Watch any corner and several attackers will be obstructing defenders rather than attacking the ball. The tactic is widespread, borrowed from other sports, and sits precisely at the edge of what the laws permit.
What the blocks are actually doing
A blocker stands in the path a marking defender must take to follow his man, so the defender has to go around and arrives a fraction late.
A fraction is enough. Attacking headers are won by the player who reaches the contact point first, and a delayed defender cannot recover in the air.
Other variations screen the goalkeeper's route to the ball or occupy the space he would step into, which is why keepers complain more than defenders do.
The laws distinguish intent, which is unobservable
Impeding an opponent is an offence when a player deliberately moves into their path without playing the ball, and that word deliberately is doing enormous work.
An attacker who simply stands still, or who appears to be moving towards the ball, has a defensible account of what he was doing.
Referees are asked to judge the intention of a stationary player in a crowd of twenty during a two-second sequence, which is close to impossible.
Defenders responded by initiating contact
The defensive answer is to make contact early, before the block can be set, and to move through the blocker rather than around him.
That escalates the physical exchange in the area, which is one reason corners now involve so much grappling before the ball is delivered.
Because both sides are holding, referees face a situation where penalising one offence means explaining why the other was ignored, so most are left alone.
Where officials do intervene
The clearest offences are blocks on the goalkeeper, because keepers have specific protection and the obstruction is easier to see in isolation.
Blatant holding that pulls a defender off his feet also gets punished, particularly since video review made these incidents reviewable in slow motion.
What survives is the subtle version: standing in a path, delaying a step, or occupying a space without visible contact at all.
Why it will not be legislated away
Writing a rule that separates a legal position from an illegal one requires describing intent, and every attempt to do so has produced something unenforceable.
Refereeing conventions have instead settled on a tolerance level that both sides understand, which is why the same actions are punished in one competition and ignored in another.
Teams calibrate to whatever tolerance they expect, and set-piece coaches study officials as carefully as they study opponents.