Serious ASIC question: Why aren't all (ASIC resistant) coins just multi-algo?

[neatneet]

inb4 ASIC resistance is a myth, i'm aware of that. That being said:

I remember when XVG was smashed by litecoin ASIC miners back in the day, and the multi-algo solution seemed to work decently to keep their coin "ASIC resistant." With the Z9 on the way, i'm curious as to why most coins don't just add algorithms to their chain as XVG once did. This isn't a shill of that coin by the way, i'm just genuinely intrigued as to why that sort of solution isn't implemented for more chains. Is it technically difficult to implement? Does there need to be the right PoW environment (enough GPU miners to compete with ASICs to even out the difficulty)?

Can anyone shed some light on this topic?