I’ve come across this as well, and having worked in VoIP the common rationale is fraud prevention, which is a real problem that does exist-but I think it’s a rationale that has been quite misapplied or perhaps better to say, inappropriately applied as a blocker for using VoIP numbers for 2FA.
In my experiences the vast majority of websites allow it -- a select few disallow. Although that group will expand. One trick I've found is to register the 2FA with a "real" number, and then port that number to a voip service. I never use a real cell phone number for anything.