Two primary reasons companies go public 1) cash out and 2) raise capital. I don't think Air BNB or Uber need money currently and I suspect they don't want the additional scrutiny/headaches an IPO brings to their businesses yet.
Airbnb last raised money through equity in March:
https://www.crunchbase.com/organization/airbnb/funding-round...
Uber last raised money through equity last June
https://www.crunchbase.com/organization/uber/funding-rounds
(but has taken on other financing since then).