Why did the oval shield replace the scutum?

[ValentinianVictrix]

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?

Robert, surely Vegetius tells us the length of various Roman spears?-

'Their offensive weapons were large swords, called spathae, and smaller ones called semispathae together with five loaded javelins (martiobarbuli) in the concavity of the shield, which they threw at the first charge. They had likewise two other javelins, the largest of which was composed of a staff five feet and a half long and a triangular head of iron nine inches long. This was formerly called the pilum, but now it is known by the name of spiculum. The soldiers were particularly exercised in the use of this weapon, because when thrown with force and skill it often penetrated the shields of the foot and the cuirasses of the horse. The other javelin was of smaller size; its triangular point was only five inches long and the staff three feet and one half. It was anciently called verriculum but now verutum.'

Your statement about Roman spears is not entirely correct. The contos in Late Roman literature is nearly always a long spear, usually the arm of the heavily armoured Catafractarii/Clibanarii, some depictions of it show it was at least 8 feet long. Whilst there are mention of many types of spears in Ammianus, such as the Hasta, Lancia and 'pilis' (pilum), there are also references to contemporary weapons such as the Spiculum and Veruta that Vegestius spoke about. Some Late Roman monumental works show infantry with a spear that appears to be about six feet to six and a half feet long, yet others, such as the pen and ink drawings of the mostly destroyed Column of Theodosius show the infantry in many cases just holding a short spear, probably the Veruta. I believe references to the Spiculum are also to be found in the SHA.