Think of it like solving an algebra equation. The fewer variables there are, the simpler the equation, and the easier it is to solve. Every additional variable makes the equation more complicated.
Tightness removes variables in your movement. When your body is loose, it moves in ways that are more complex and harder to predict; being tight means control over everything your body is doing, making the skill more predictable and more reliably replicable.
Sports like soccer are less reliant on very precise, practiced movements, and more reliant on things like running and jumping, with improvised changes in direction and speed. These movements are far more mechanically complex than a cartwheel, but fortunately our bodies and brains have spent millions of years learning to do each of these things as efficiently as possible, so in these cases it's best to just relax and let the body do what it already knows how to do.