By comparing a $3000 rent-a-ride day in an F2000 to purchasing a kart engine, you’re not really comparing apples to apples. A full arrive-and-drive entry at a national level kart race would be 1/3 of that probably.
The first answer would be the manufacturers are throwing money at the series to run their engine only.
But also, if you were actually trying to compete across the entire season in a mixed engine TaG class, you needed multiple engine makes to consistently run up front, as the track layouts favored a different engine each weekend.
Yes, it does suck for the weekend warrior or local guy who wants to come try his hand but doesn’t have the right engine, but the manufacturers do bring in some nice incentives for karters, and the playing field in the TaG classes is much more level and even now than it ever was.
Also, there are still 100cc classes out there if you’re looking for something cheaper to run.