Kernel is not a monolithic application, and you cannot develop it like one. There are tons of actors: independent developers, small support companies (like Collabora), corporations, all with different priorities. There is a large number of independent forks (e.g. for obscure devices), that will never be merged, but need to merge e.g. security patches from the mainline. A single project management tool won't do, not your typical business grade tracking&reporting tool.

CI is already there. Not a central one—again, distributed across different organizations. Different organizations have different needs for CI, e.g. supporting weird architectures that they need to develop against.

There is a reason Torvalds created git—existing tools just wouldn't work. There might be a place for a similar revolution regarding a bugtracker…