Because while hilt slashing may be fast, it isn't instant. Those "instant" hits, are something else entirely. A hilt slash is simply hitting outside of your sweet spot, normally this can cause glancing, but because of the footwork that is generally used in conjunction with this, you're easily able to keep yourself from glancing due to the added speed bonus. It can quite easily be blocked, but it's a skill you have do have to learn. I could go on, but Its actually harder for me to come up with reasons for why it is bad(without being cynical) than reasons for why it should stay. Maybe there's an argument I have yet to hear on the subject that could change my mind.
The fact that hiltslash hits fast isn't the problem, it's that this speed allows people to get a "free" double hit (after their first attack is blocked) if they properly guess the direction of their opponents' next swing. Against a 1h for instance, the hiltslashing player knows that the next swing is likely a left-to-right swing (since 1h thrust sucks, 1h right swing is slow/glance-prone, and all overheads are risky), so after their attack is blocked, they can just strafe left while executing a left-to-right swing and turning into it for a free hit. 2h hiltslash can land these double hits even with a slight weapon speed disadvantage (i.e. a Danish user probably can't do it against a Scimitar user, but a Miaodao user has no problem).
The problem is that attempting this maneuver is a risk-free proposition; if they guess wrong on their opponent's next attack direction, they just cancel their swing, block, and try again next time. I have a problem with any mechanic that allows high reward (a potential "out of turn" hit in this case) with no potential drawbacks.
If it can't be fixed by animation/sweetspot tweaks, one idea would be to disallow feints on any swing immediately following up a blocked swing. i.e. make it so that if someone is "spamming", they have to commit to the risk associated with said "spam" (that they may get hit during their followup swing).