What is the point of Layer 2's on top of Ethereum?

I don't really get the point of L2s. They're supposed to help Ethereum scale, right? But at the same time the user experience is like using an entirely different network anyway. You have to set up a totally new RPC endpoint in Metamask/whatever wallet so why not just make different scalable L1s that have nothing to do with Ethereum at that point? I just don't understand what the point is of complicating things by requiring Ethereum as a base layer.

If the point is to inherit Eth's level of security by periodically settling transaction batches on it, then I still don't get why these exist. If these were separate L1's and they got popular enough for it to matter, they would still easily attain sufficient security through a large number of miners or validators.

Can anyone explain what I'm missing? They just seem like Layer 1's with extra, unnecessary steps.