Because they had to draw a line somewhere? Is that the answer you're looking for? I guess they figured it was reasonable for a pet to be able to be able to double/triple attack, have a high rate of critical hits, or be drawn to shiny things (Treasure Hunter) while it would be unreasonable for a rabbit to have a Great Katana strapped to its head. Not to mention, samurai rabbits have already been done.
Double Attack, Triple Attack, Evasion Bonus, Magic Defense Bonus, etc. are powerful as well. Should BST pets have any traits at all? After all, every trait they could have is something players have access to naturally or have to sub for. Why is this only a problem when it's a THF trait? Most pets are WAR. Should WARs (including myself) be offended and rage against the BSTs? What makes THFs so special that their job traits are off limits while everyone else is fair game?TH especially is an extremely powerful trait, more than most people give it credit for. TH2 over TH1 means you only have to kill two mobs instead of three, on average, to get your drop. Thus it saves you about 33% of your farming time (on single drops, unaffected by weakness staggers or anything other than TH). That is a huge difference. One that's worth subbing THF for, if you wanna reduce the amount of farming you want to do for a specific drop.