Why doesn’t wifi switch to data when it’s slower?

Simply turn off data saver and go into the developer options menu and enable "aggressive WIFI to cellular handover"

Simple.  Wifi is free.  Data usage cost you money.  The first time someone's phone switched to data, uses up all their hours, and then runs up a bill for overage on data, the providers (and the makers of the phone and the developers of the OS) will get sued.  By making you have to manually switch over they can not longer be held responsible for you running up a bill.

wifi doesn't cost you anything (typically). most people don't have unlimited data.

Lots of reasons. "Data" (by which you probably mean 4G/5G) usually costs money, for one. Two, it would have to test downloading over the other connection first to know it was faster. Three, switching connections would interrupt any current downloads. Four, the content might not be available on the other network (data from your LAN, for example).