Protective but flexible musculata.

[Dan Howard]

Whilst there have been finds, I doubt very much if more than 5% of all the armour and equipment worn has been found, probably more like 1 or 2%, anyone willing to bet on that percentage as being representative of all the armour worn?

Not just "finds". There are literally tons of surviving leather artefacts dating to the Roman period including shoes, shield covers, tents, clothing, cloaks, belts, straps, scabbards, horse tack, even leather fittings for metal armour. By your logic the Romans must have had a magical leather that disappears when it is made into armour yet stays in the ground for centuries if it is made into other things. Amongst all these leather items, and there are lots and lots of them, we have one single surviving piece of armour - some lamellar from Dura Europos. There is more than enough evidence to conclude that Roman leather armour was rare.