Flankers need to be able to get in and get out to be effective. If you're playing d.va you pick one of those two things.

Fly in and you need to use DM's full duration (and then some) to survive if there's more than one person around. If you do survive then DM's on cd for when your team actually needs it.

Walk in with the intent to fly back out one you secure a kill and the enemy team just walks away from you. You move too slowly while firing to catch up to anyone so you never get the pick.

D.va is good aggressively when pushing with a Lucio boost (and the rest of her team) which lets her save boost for when she actually needs it, rather than burning it to engage. Dva is good defensively from behind the Rein shield with an Ana behind her.

If you're saying this because you play d.va like that, you're playing against teams that are awful and have no idea how to punish out of position enemies. If you're saying this because d.vas play this way against you, your teams are trash at punishing out of position heroes.