LRMs have low velocity. They take a long time to reach their target.

The time required to lock onto your target before firing LRMs is time that your target can shoot freely at you.

The damage dealt is not focused. 1 damage per missile rains all over the target, chipping at arms, torso components, and legs alike. 2LRM20 sounds impressive until you divide 40 damage by 10 components (front+rear LT, front+rear RT, front+rear CT, RL, LL, RA, LA) and note that an opponent who torso twists took about 4 damage per component assuming all LRMs hit.

Anything direct, like 5MPL for example, is capable of dealing that same 40 damage to a single component in the time it takes LRMs to lock on.

Add to this the request for holding locks and it's no surprise people get upset. When you ask to hold a lock that means you aren't trying to get them for yourself. You're using your team as armor which means at best, in a face to face fight, your team is fighting 11v12 + some inaccurate, low damage fire support.

Nothing about LRMs encourages a healthy play style. Too many newbies pick them up and become noobs.