Learning Java once is easy. Understanding it is fine too. But learning new framework every time you switch projects - not so much. Because every time it becomes a gargantuan task, with all the dependencies that it pulls. And the moment you wants to do a tiny step aside, you need to learn intricacies of internal working of said framework just to something oh-so-slightly different than original authors envisioned. And then add couple more libraries, and endless conflicts, and stubs, and injection debugging nightmare. The maintenance of the dependencies becomes a full time job...
Yeah, Java is simple. Everything else around it makes it almost unbearable with mental load that you need to keep just to move forward with a simple change.