Because PeerTube servers are independently owned and run, they can't have the rug pulled from under them the way Vimeo and YouTube have now done.
A homegrown server is entirely yours and under your control. No one can suddenly take it away or demand a bill for thousands of dollars.
A few years ago, YouTube decided to shut down the software maker Blender's channel without warning or explanation. Fortunately, Blender also have their own PeerTube instance to use instead: https://video.blender.org