Tend to happen when there is jack shit to do on main (also not 100% true, there was also legendary questline and people min-maxed garrison for free pieces of gear from current or even higher difiiculty you completed). That was good side effect of horrible and outdated design, not good design.
Of course system where extra stuff to do like in Legion (so people with 1 char has some content for character progress), but completely account-wide (so people with alts have their freedom) would be superior to both older and modern design. But it was Legion, right now in 9.1.5 we are pretty close to that, especially if new stuff from 9.2 will be either account-wide (flying, 2nd lego) or not relevant for progression.