One reason, to avoid the scrutiny describing their actual business would expose them to more scrutiny, e.g., Uber is transportation company, not a software company.
Once the BBC described Google as an "American advertising company", now that's cold.
Uber is not a bus company. It does not own a gigantic fleet of cars or large depots to store them in, nor does it employ mechanics and cleaners to tend to them, or have long-term contracts for gates at stations around the country.
Uber is a tech company because it really only owns software and lawyers. The cost of bringing 10,000 new vehicles on the road is minimal -- they simply increase the incentives to drivers in that area.
Likewise, Air BnB is a tech company, for the same reason.
WeWork, however, is simply an office real estate company. Its business model is closer to a hotel chain.
Ironically, if Uber achieves its self-driving dreams it will have acquire all of these things.