Because of how lucrative European football is in terms of revenue and transfer power, it's always better to be a big fish in a small pond.
There's a not too dissimilar situation in the Welsh league. There's one full-time professional club and they've been a dominant force for over a decade; TNS. They've won 9 of the last 12 titles including a current run of 5 in a row. They've also won six doubles and two trebles in that time.
TNS are based in Oswestry (in England, ironically) which isn't too far from Wrexham and the TNS top brass often like to tease that they could beat Wrexham seeing as how our misfortune sees us in the Conference but would TNS ever actually join the English league and work their way up from the Shropshire county league? Of course they won't. Why would they? Why would they give up their golden goose of titles and cups and European runs to be slugging it out in the English lower leagues?
And why would Celtic and their plus one, whoever it may be? Sure, they'd get a shortcut and be entered into the Football League from day one and even if they are several classes above TNS and any English third tier team, there's no guarantee that they would even make it to the Premier League and even if they did, I don't think they would be good enough to compete for the English title.
Celtic (and Rangers) can draw players in not only due to their history but also due to the opportunity and challenge of playing in the Champions League and Europa League. That is a huge selling point and that would disappear with a transfer to the English leagues. It would be foolish to give that up.
As for fixing this:
That's another matter entirely.
