FPS: First Person Shooter.
Melee should be a last resort, not an optimal tactic.
I particularly dislike knives in modern-day FPSes since they're so much easier to use effectively than guns. A bullet has a very small margin for error; whereas in most games, a knife simply requires pressing a button while facing an enemy (within melee range).
Combine the above with multiplayer games where the maps are largely built around close/medium-range combat with limited lines of sight. In theory this makes fast-paced (ranged) firefights involving ducking in and out of cover and regenerating health, but in reality, this design philosophy makes it easy for melee-ers to close distance to ranged attackers without dying. If both playstyles are viable, the easier one will typically be the most popular. Hence the abundance of Commando + magnum + tactical knife in MW2.
Furthermore, given the RPG elements of most multiplayer FPSes, one can improve one's melee combat skills but given that melee is already a one-hit kill, it is typically range and attack speed that get buffed; despite not really needing it.
I blame Bobby Kotick. I'm sure it's his fault somehow.
With that said, I enjoy melee in single-player FPS gameplay since it feels more visceral and dangerous. Sneaking up behind someone and knifing them carries more risk than headshotting them from cover, but it requires less skill, so I'm ok with it. The "Thief" games got this right, IMO: people shot anywhere other than the head took a long time to bleed out and die, and could alert others. By contrast, a knife/blackjack to the head from melee and they died *instantly*, and the body was right in front of you, ready to be hidden. The risk was directly proportionate to the reward and the skill was more in choosing the correct strategy for a given situation, rather than simply the execution .
Pun intended, sorry.