Co-founder of Levels.fyi here - it's not so much that these sites solely support FAANG companies, it's just harder to get other people to add their salaries. We're working to encourage folks at other companies to contribute through dedicated company pages [1], etc. It's had a real impact and we've started to collect much more data across more companies / roles. We're also working to incentivize contributions more and I do think we'll be in a position to have a more representative sample over the next few months.
[1] https://www.levels.fyi/company/