There is a major downside to encryption: If you forget your password or your tpm fails and you’ve not backed things up, then that data is gone forever. If someone doesn’t have anything incriminating or useful to theives on their device, the easier reparability might justify not enabling it.
Android backs up data to the cloud. If the phone breaks or gets stolen, you don’t need to recover data from it - you can just pull it from Google’s servers.
In addition, people tend to not treat their phones as “permanent storage”. The concept of losing or breaking their phone is probably more clear, so they make sure to back it up in some way to the cloud or their desktop.
Also, it’s much more likely for a phone to be stolen than a laptop or desktop.