“But in the deep sea” *slaps you*
These creatures evolved for environments near the surface. We’d have seen them. Ever seen what happens when you take a creature to the wrong level of pressure?
Hint: the blobfish isn’t actually supposed to look like that. What you’re looking at there is the oceanic equivalent of dragging a human into space and shoving them out an airlock
To add to that, there are deep sea sharks but they’ve evolved to thrive in low energy environments. Not much energy arrives down there. So they’ve evolved to metabolize slowly (hence the 400 year lifespan).