Wow, surprised to hear this! I had assumed in a project that large and longstanding, most of the bugs would have been smoothed out.
A lot of these bugs seem to also go unfixed, I think with the intention that you pay for Unity Pro which gives you source code access to the engine and you can work around them / fix them yourself.
I don't think they are deliberately leaving bugs unfixed in order to up sell people though. More its just an enormous software project with an awful lot of people using it in lots of different ways. Basically an enormous surface area to try to tackle.