Can someone explain why layer 2’s need eth?

I got into crypto about 3 years ago but I’m still trying to understand ethereum technicals

I get eths value in being a programmable asset , maybe the first mainstream one but, if it relies on users to use layer 2s like polygon for scaling and lower gas fees, wouldn’t people just flock to the layer 2s?

Like isn’t polygon just doing eth’s job ?

Not trolling, just confused.