The rising air doesn't actually stop at cloud base, but it becomes very hard to orient because you don't see the horizon anymore. The thermals can also be generated by many different landscape features - they can be triggered by hills (which can also help them release) but there are plenty on flat landscape as well. Also other sources of upward air currents can be used for soaring, e.g. wind deflecting upwards over a hill or waves high in the atmosphere (search "wave soaring" on youtube). There is even https://perlanproject.org/ aiming to do this at 30km altitude.
The other reason gliders stop at the cloud base is that it's illegal to fly close to the clouds in most airspace. Because it makes it very hard to avoid collisions.
Good point. I oversimplified. Birds and pilots tend to not fly up into clouds for visibility reasons but the air keeps rising into the cloud. From experience, it's a surreal experience riding a thermal up into a cloud in a paraglider. Just not safe or legal in most places to stay in the cloud.
