Gail Williams is dead on here (as usual!) I have encountered many students over the years who are constantly looking to work on the most difficult excerpts and solo repertoire before putting in the work to be able to play their instrument well.
It takes discipline to play the first page of the Arban's book in multiple octaves every single day while making every note identical to those around them no matter the octave or the dynamic. It is easy to do that kind of work every once in a while. But having the in-the-moment discipline to know that you need to be doing that kind of practicing on a regular basis is what separates the good players from the great players.
As I once heard Joe Alessi say, it takes a lot more work to obtain skills on your horn than it does to maintain skills on your horn. Gail Williams spent an insane number of hours being able to play loudly in all registers with a good sound and a variety of attacks, releases, weights, etc. Everyone who can charge people a good chunk of change to listen to them play their instrument has done that work.
Furthermore, she doesn't have to spend nearly as much time today practicing that stuff. I'm sure she still spends more time than you might believe, but it pales in comparison to when she was first acquiring those skills.
So if you are a young tuba player, rather than jumping into the John Williams Concerto your freshman year, maybe do the sometimes tedious work that your teacher suggests on a regular basis. I promise you will be able to play the John Williams before you know it!
Fundamentals before fireworks!