Why did the oval shield replace the scutum?

[Robert Vermaat]

In fact, most shield walls, whether Celtic, Greek, Persian, Roman, you name it, used spears 2.2-3 meters long.

Yep, and spears were pretty much the same, too, given varied length for function, etc.

Where do you guys base that on? Roman pila were c. 6 ft (1m80), and these were seen as too short from the early 3rd c. onwards. Did you know that no Roman spear has ever been found?Did you know that, based on funerary monuments, no Roman spear would have been roughly longer than the height of a man?
How long were Celtic spears? Where do you find that information?
How long were German spears in your opinion, and where do you base that on?