Because while an ultimate getting 1 kill is better than getting no kills, the function of an ultimate is to drastically change the pace of a fight. If I can make a fight 6v3, I'm not gonna blow my load on using it early to make the fight 6v5, if one of the 2 survivors on their team has an ult to make it 5v5 thereafter. Graviton surge isn't for one person kills, it takes too long to charge.
Tracer maybe, aight, her ult charges fast enough to blow on solo kills. Similarly though, if we're defending I'm probably not going to use Valk just to get off a single instant rez. Especially if you're a relatively expendable dps, and not a point-holder like tanks or sustaining heals. I'm sorry but that's how it is, ults are too important to blow when there isn't a proper team fight in place or the enemy isn't positioned in a way to be deeply affected by it.