Most teams invert their fullbacks because they saw someone else do it

It is a specific solution to a specific problem, and roughly half the sides using it do not have that problem.

Inverting a fullback works when your opponent presses with two forwards and you need a third body in the first line to break it. That is the problem it solves. Two press, you build with two centre-backs and a pivot, they cover the pivot with a midfielder stepping up, and suddenly it is three against three with no free man. Bring the left-back inside and it is four against three. Ball goes through. Lovely.

Now imagine the opposition are not pressing. They are sitting in a mid block with a compact midfield four, perfectly happy for you to have the ball on the halfway line, waiting to spring. Your fullback comes inside anyway, because that is what the team does, and now you have five players in a congested central area, no width on that side apart from a winger who is being doubled, and if you turn it over the counter has a motorway.

That is the picture week after week in leagues below the top handful of sides. The tactic applied because it was seen, not because it was needed.

The profile requirement is the other bit that gets skipped. An inverted fullback is not a fullback standing somewhere different. He needs to receive on the half turn in traffic, be comfortable with a forward pressing his blind side, play a line-breaking pass under pressure, and have the defensive sense to know when to abandon the whole idea and get back out wide. That is a midfielder's skill set in a defender's body. There are not many of them. A right-back who is fundamentally a good crosser and a strong one-against-one defender is made worse by this, not better.

What I would rather see, and occasionally do, is inversion used situationally rather than structurally. Invert against pressing sides, hold width against blocks, and let the fullback read it in the moment from where the opposition forward's shoulders are pointing. Some managers coach exactly this. It is harder to coach than a fixed rule and much harder to explain in a graphic, which may be why the fixed rule keeps winning.

The position is not a metric and it is not a philosophy. It is a tool. Half the league is using a screwdriver on a nail and calling the result modern.