Around here, my impression is that tubes are considered distortion generators and little else. In my time with tube amplifiers I remember reading that there were certain things that tubes do better than solid state components. I know nothing about electronics but am interested in whether there is any truth to that, and if so, could whatever that is be incorporated in circuit design.
Tubes and transistors (including FETs and MOSFETs) are are all
"similar" in that they allow you to control a large voltage and/or current with a much smaller voltage/current, In other words, it's an amplifier.
An analogy is the gas pedal in your car. The pedal controls fuel to the motor to control speed of your car. The muscles in you leg aren't moving the car.
Any of these things can give you good sound but the older technology is more expensive (especially if you want good sound) and tubes are much less energy efficient. An op-amp costs a couple of dollars (or less) and it can contain hundreds or of transistors.
Tubes are also less stable. As the tube ages the characteristics change. That can be designed-around (with negative-corrective feedback, etc.). But some tube designs are intentionally designed to sound different when you swap tubes and that's not a stable design. Transistor/MOSFETs also very from part-to-part so again feedback (etc,) is used to get predictable performance, but sold state parts don't "age".
Jim mentioned output transformers (for power amplifiers). Again that's a very expensive item if you want flat frequency response and low distortion, and the more power you need the bigger and more expensive it gets. Some preamps use input & output transformers but they can be built without them. The smaller transformers also aren't cheap but they are more affordable than a power-output transformer. (Low-power audio transformers can be reasonably priced if you don't need good sound quality, but it's still an added expense.)
Overall the cost-per-watt is MUCH lower with solid state. And there aren't a lot of high-power tube amps. It's easy to find 1000W (or more) solid state amps but I don't know if I've seen a 500W tube amp,