Mass (weight), height, length (wingspan), inline speed (forty), agility (shuttle/cone), explosion (jumping), strength (bench/weight room data)…. all measurables matter.
Do some fans overrate forty times in a vacuum… for sure. But a great forty time paired with great accompanying measurables is what a scout is looking at… along with brains and football smarts.
Randy Moss was tall, long, explosive, had good agility for a tall guy and was really football smart… but his speed on top of these things made him dominant.
In a league full of fast people, Moss was the fastest… and by chance you ran with him, he could out leap and out stretch you.
There’s alternative versions of this where agility for example was the defining trait. Look at Stephon Gilmore for one, he was fast and had all the measurables, but what made him unique was at 6 feet tall he had absurd agility, he had the quickness of a much smaller player. His 3.94 shuttle and 6.61 cone were absurd for his height.
Measurables and speed aren’t all that matter, but they matter a lot.