There's issues the vendors and everyone have to deal with equally.
Not speaking for them but it's hard to justify carrying inventory and buying an entire production run in cash to the subcon machine shop manufacturing them if the projected sales might not show the money coming back for years.
Shops and builders are having a tough time with some lines of parts being in stock and the niche stuff isn't all over the country.
In addition to that many rebuilders have shut down and the remaining ones have very long lines at times.
Some things won't get a place in line without an up front payment.