Why do we call it a 51% attack instead of a 50% attack? From the Bitcoin paper:

p = probability an honest node finds the next block
q = probability the attacker finds the next block
qz = probability the attacker will ever catch up from z blocks behind

qz = 1 if p ≤ q


In other words, when an attacker controls 50% or more the success rate of a double-spend attack is 100%.

Why is it referred to as 51% attack, and not 50% as 50% is also enough?

Am I missing something here?
The distinction is of theoretical importance only.   But if the attacker controls exactly 50%, then it's true that the attacker will eventually catch up, but he won't stay caught up: the honest population will eventually overtake his chain, and we'll be in an unstable situation where control of the "best" chain will bounce back and forth between them forever.

If the attacker wants to eventually catch up and stay caught up, he needs to have strictly greater than half of the mining power.  In this case, they might bounce back and forth for a while, but eventually there will come a time when the attacker takes the lead and never again loses it.  We say "51%" informally to summarize this, though of course 50.1% or 50.000001% would also be sufficient.