Magnetrons are cheap, reliable, and work well enough. Their one and only job is to produce energy at about 2.4GHz, and to make a lot of it. You merely put in electricity and through the magic of geometry you get RF out.

A transistorized solution is, simply, inappropriate. The RF section alone will cost more than an entire microwave off the shelf, and plus that you need all the control circuitry and high speed crap to generate a 2.4GHz tone. Not to mention that putting kilowatts through silicon creates a much higher risk of catastrophic failure: transistors blowing out into a plume of plasma and such.

You're asking why we haven't reinvented the wheel. There's simply no improvement to be had.

You want high power RF in a given frequency band, you want a magnetron. It's just a lump of iron with some wires. You don't need any electronics. At all.