Technically, everything in the game is by design. In this case, however, I think it’s more an oversight.
We used to have burst. Aspect of the Wild and Bestial Wrath were meaningful cooldowns when Kill Command did significantly more damage than our other abilities.
Then Azerite traits and Corruption came along and we got lucky with those. Our Azerite traits were significant and stacked well together. They made our pets incredibly strong. Corruption, on the other hand, made us incredibly strong. Instead of being two halves to a whole, we were essentially two different toons doing damage. In response to that strength, our burst was toned down. We were still very strong, because sustained damage was important in BfA.
Additionally, because our pets were so strong on their own in BfA, there was a lot of complaints on the forums about Hunter pets. As stated above, our pets were only that strong due to Azerite traits, but Blizzard went ahead and nerfed our pets anyway. Spirit Mend was obliterated, survivability was demolished, and revive pet had it’s cast time doubled. That’s a huge nerf considering the mechanic that made them strong in the first place was removed from the game.
Then Shadowlands comes along, where the meta in every aspect of the game is burst. Raids? Burst. Mythic+? Burst. PvP? Burst. Unfortunately, BM was “balanced” around Azerite traits and Corruption, which are no longer a variable, and the spec wasn’t compensated.
Blizzard has stated they don’t want to change the rankings for DPS in the middle of a patch and it will be looked at in 9.1. However, I find this claim to be suspect, because multiple specs have risen and dropped on the ladder over the course of 9.0. It really just sounds like an excuse to do nothing.
As for our mobility being the trade off for burst, we already know that isn’t supposed to be the case. Our mobility is our trade off for lack of defensive cooldowns. Hunters are arguably the squishiest class in the game. Rather than reducing damage or healing it, we’re supposed to get the heck out of the way.