Because people do pay the money.

If an item is hard to find they will pay it.

People that only buy stuff on eBay will pay for it.

If someone randomly sees the item they need they will buy it.

Example: I paid $2,200 on eBay for an RTX 4090 FE because they were sold out everywhere. The extra few hundred bucks for me was worth it to get the FE card over 3rd party cards since it is the smallest and quietist of the bunch. None of the other 4090s fit in my case.