Here's your statement:
That's definitely true, if you think that over a quarter of the tracks in the game by your criteria count as "we don't have"
See, "everything the game has" is only 58 tracks, and that's all variants, including reverse. Here they are:
Alsace Village & Reverse
Autodromo de Interlagos
Autodromo Nazionale Monza
Autodromo Nazionale Monza (no Chicane)
BB Raceway & Reverse
Blue Moon Bay Speedway & Reverse
Blue Moon Bay Speedway - Infield A & Reverse
Blue Moon Bay Speedway - Infield B & Reverse
Brands Hatch GP Circuit
Brands Hatch Indy Circuit
Circuit de la Sarthe
Circuit de la Sarthe No Chicane
Colorado Springs Lake & Reverse
Dragon Trail Gardens & Reverse
Dragon Trail Seaside & Reverse
Fishermans Ranch & Reverse
Kyoto Driving Park Miyabi
Kyoto Driving Park Yamagiwa & Reverse
Kyoto Driving Park Yamagiwa+Miyabi & Reverse
Lago Maggiore Center & Reverse
Lago Maggiore East & Reverse
Lago Maggiore GP & Reverse
Lago Maggiore West & Reverse
Mount Panorama
Northern Isle Speedway
Nurburgring 24h
Nurburgring GP/F
Nurburgring Nordschleife
Nurburgring Nordschleife Tourist
Sardegna Windmills & Reverse
Suzuka Circuit
Suzuka East Course
Tokyo Expressway Central Inner Loop
Tokyo Expressway Central Outer Loop
Tokyo Expressway East Inner Loop
Tokyo Expressway East Outer Loop
Tsukuba
Willow Springs Big Willow
Willow Springs Horse Thief Mile & Reverse
Willow Springs Streets of Willow Springs & Reverse
How many short tracks there are depends what you define as "short". If we use the sub-two mile definition you seemed happy with above, 19 tracks qualify. That's 33%, and they are:
BB Raceway & Reverse
Blue Moon Bay Speedway & Reverse
Blue Moon Bay Speedway - Infield B & Reverse
Brands Hatch Indy Circuit
Colorado Springs Lake & Reverse
Kyoto Driving Park Miyabi
Lago Maggiore Center & Reverse
Northern Isle Speedway
Suzuka East Course
Tsukuba
Willow Springs Horse Thief Mile & Reverse
Willow Springs Streets of Willow Springs & Reverse
However, that's a faintly ridiculous way to count tracks.
Let's ignore the rally tracks (because Polyphony largely does too), and the three ovals (Northern Isle is only short because it's short; this seems like an odd thing to say, but it's a half-mile banked oval and there's nothing really that would distinguish it from a five-mile banked oval like Test Track). Let's dump the reverse tracks too - including Inner/Outer Tokyo variants, which are reverse tracks, but distinct on the grounds of using the opposing carriageways - and the "No Chicane" versions of Monza and La Sarthe, plus the Ring's Tourist layout (you can't race on it, and it's just a different start/finish line).
That gives us 29 tracks:
Alsace Village
Autodromo de Interlagos
Autodromo Nazionale Monza
Blue Moon Bay Speedway - Infield A
Blue Moon Bay Speedway - Infield B
Brands Hatch GP Circuit
Brands Hatch Indy Circuit
Circuit de la Sarthe
Dragon Trail Gardens
Dragon Trail Seaside
Kyoto Driving Park Miyabi
Kyoto Driving Park Yamagiwa
Kyoto Driving Park Yamagiwa+Miyabi
Lago Maggiore Center
Lago Maggiore East
Lago Maggiore GP
Lago Maggiore West
Mount Panorama
Nurburgring 24h
Nurburgring GP/F
Nurburgring Nordschleife
Suzuka Circuit
Suzuka East Course
Tokyo Expressway Central
Tokyo Expressway East
Tsukuba
Willow Springs Big Willow
Willow Springs Horse Thief Mile
Willow Springs Streets of Willow Springs
And of those 29, eight qualify under the two-mile rule:
Blue Moon Bay Speedway - Infield B
Brands Hatch Indy Circuit
Kyoto Driving Park Miyabi
Lago Maggiore Center
Suzuka East Course
Tsukuba
Willow Springs Horse Thief Mile
Willow Springs Streets of Willow Springs
And that'd be roughly 28% of tracks.
But... you gave us a new rule:
Trial Mountain is 2.47 miles long in GT6 (just under a quarter of a mile longer than Deep Forest). With that as the minimum length, we now have 12 qualifying tracks:
Blue Moon Bay Speedway - Infield A
Blue Moon Bay Speedway - Infield B
Brands Hatch GP Circuit
Brands Hatch Indy Circuit
Kyoto Driving Park Miyabi
Lago Maggiore Center
Lago Maggiore East
Suzuka East Course
Tsukuba
Willow Springs Big Willow
Willow Springs Horse Thief Mile
Willow Springs Streets of Willow Springs
That's 41% of tracks now.
But wait, there's more! You apparently include Dragon Trail Gardens as a short track, and that's 2.7 miles long!
This minimum length increases the count to 15 - FIFTEEN!
Autodromo de Interlagos
Blue Moon Bay Speedway - Infield A
Blue Moon Bay Speedway - Infield B
Brands Hatch GP Circuit
Brands Hatch Indy Circuit
Dragon Trail Gardens
Kyoto Driving Park Miyabi
Lago Maggiore Center
Lago Maggiore East
Lago Maggiore West
Suzuka East Course
Tsukuba
Willow Springs Big Willow
Willow Springs Horse Thief Mile
Willow Springs Streets of Willow Springs
15 tracks is more than half - 51.7%, if you're interested. So now most tracks are short ones! However, there's one more qualifying criterion you mentioned:
Deep Forest (home straight) and Trial Mountain (back straight) both have a straight measuring roughly half a mile. Willow Springs - which you were happy with earlier - is about the same, as is Suzuka East. Interlagos is, surprisingly, shorter, at about 710 yards (0.4mi), but that's because there's a corner right at the pitlane entry. Brands Hatch is much shorter, although including the kink at Pilgrim's Drop, it comes in around 740 yards (0.42mi)...
I'd have to get out and measure the straights at Lago Maggiore, but the longest one, on the GP and West circuits doesn't seem as long as Dragon Trail Gardens' main straight - and you're perfectly happy with that track.
So we could potentially ditch only the two Blue Moon infield tracks, leaving 13 of 29 tracks as "short", by your criteria. Even if you want to include the ovals and rally tracks, it's 17 of 35 (and that's almost half).