Because until border bounces are possible, they will lose too many foreigners if they remove the Covid extension. You may think that there are a lot of tourists in Thailand right now, but no, very little still. They can't afford to annoy the ones who still rely on Covid extension. When border bounces are back and buses go to borders every day, they will remove Covid extension. For now, people are flying to Phnom Penh to get tourist visa, it's simply not efficient enough so such Covid extensions will remain until border bounce reopens.
Just avoid Suvarnabhumi Airport, enter from Phuket, Chiang Mai, etc. Or if you enter via Suvarnabhumi make sure that you have an agent phone number to call to bribe your way in if they want to denied u.