I'm a trumpet player who's played in many orchestras.
The first thing to understand is that historically, the trumpet is a relatively new addition to the orchestra. Before the mid-19th century, metalworking wasn't sophisticated enough to build valves, so trumpets from before this time were more like bugles, unable to play fully chromatically. With a limited set of notes, the instrument was necessarily limited to a supporting role. And especially considering its ability to play loudly, combined with the softer sound of gut strings and early bow design, it just makes sense to use it only to accentuate the loud parts.
Trumpet concertos do go all the way back to the Baroque period (e.g. the famous Brandenburg Concerto no. 2), but these are sort of insanely hard and you wouldn't be able to write lines like that in normal parts.
So by the time valves got invented, the orchestra had already been developing for centuries, and the trumpet had already been designated as a supporting instrument. And if you're looking at music from the Baroque, Classical, or about the first half of the Romantic periods, then those parts were written for valveless trumpets. Mid- to late-Romantic period composers started experimenting with brass (and saxophones, and other loud metal instruments), and modern composers have utilized the trumpet quite a bit more. Consider the fantastic trumpet parts in Mahler, Holst, Copland, and Bernstein just to name a few.
I don't buy this common idea that trumpets are just so loud that they can't balance. Trumpets are very capable of playing at a modest volume. But, in a concerto, you have to be especially careful not to cover up the soloist, so orchestra parts tend to be more conservative.
if you guys don't get that many lines, how can you stand the constant sitting around waiting? Is that something that dampens your enthusiasm for playing in an orchestra?
Part of being a mature musician is understanding that sometimes your instrument is needed, and sometimes it's not. You have to be able to appreciate a sparse part, and be willing to take on a supporting role sometimes. I would rather have a sparse part where every note I have is artistically meaningful, than a busier part where much of my material is filler that the orchestrator gave me out of pity. But, it's not for everyone, and I'll admit that I'm less enthusiastic about playing the insanely sparse parts in Classical period music. It's important for orchestra directors shy of the top professional level to program concerts that involve everyone to some degree.