As
@CustomX summarized it's all about longevity, whilst it may be 'frustrating' to have to wait for the car to reappear in the rotation the formula works for keeping people playing games because they know the person will come back on and play when that vehicle becomes available.
It is not only GT7 that does this, GTA Online, Genshin Impact etc all offer limited time rotation items if you login and play for events weeks etc, whilst it will grow 'stale' for us long term players (not that I own everything from LCD) being able to just 'buy' everything at once like we had in Sport got a bit dull.
Also the dealership may well get some DLC cars in future plans so you never know the cycle may always have some new content to keep us coming back to check, it is a good strategy that works regardless of game.