Wait, there's instant kill attacks? As in "press a button and your opponents die with little to no effort or skill on your part"?
Most of the high damage ults have a delay of some kind to give the enemy time to react (like D.VA's mech, Tracer's pulse bomb and McCree's high noon), do damage over time intead of instant (like Reaper's spin thing, Hanzo's dragonstrike and Pharah's justice), or have short range (like Reaper's and Tracer's). Also there are a lot of abilites that counter high damage ults, like Zenyatta's ult which makes him invincible and heals everyone around him really quickly for 6 seconds, Mei's ice wall which blocks damage and sight lines, Mei's ice block which makes her invincible but immobile for 4 seconds (which teammates can hide behind), Mercy's resurrect (although admittedly that doesn't work if Mercy dies in the ult) and Lucio's drop the beat (which gives his teammates huge shields that wear off after about 10 seconds). (There are probably more examples I'm forgetting) Also, most of the ults slow or stop the ulting player's movement so it's easier to kill them before they kill your teammates, esp. if your team has long range characters like Widow and Hanzo who are usually far away from the rest of their team.
So using high-damage ult's effectively usually requires careful positioning and taking the enemy team by surprise, and even then they still have options sometimes depending on their team composition.
Last but not least in case you didn't know, ults need to be charged up by doing damage (they do also charge up over time, but VERY slowly). So depending on your hero and how well your doing, you can only use them once every 2-4 minutes or so.