Why do 1st Century Roman groups avoid combat

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The problem that arrises with "the real thing" is that it involves hacking to pieces a by and large unarmoured opponent in a very controled manner. That is what made the Roman army great, dicipline, training and superior weaponary.

True, but how is this different from later Roman warefare?

I believe there is still very much to be won in a good drill display, showing the whole range of Roman combat. This calls for a sizable group, not six soldiers and a bloody centurion to lead them.

True again. What kind of size do you feel is sizable?

Investing in wicker training shields and wooden swords may well add to the display, with role play of unarmoured agressor against Roman soldier. This can be performed by smaller groups, too, as the training is mainly one on one. Also, it can attain a high degree of authenticity, something just not possible in a combat display.
The public does want to be entertained, but even they will realise "the real thing" just isn't on. This is NOT football, the real thing is bloody war. And they "know", thanks to Hollywood, what a "real" battle looks like. Blood, gore and guts, limbs hacked off, heads split. Charges decimated by pila and arowshowers, splintering on shield walls, the opponent trodden down in a controlled and massive advance, the fleeing survivors ridden down by cavalry.
Ten Romans taking on fourteen Celts/Dacians/Carthagians just doesn't cut the mustard, it quickly turns into a dangerous one on one melee and does not reflect "the real thing" of battle in the first century

Again, I do not really see how this differs from later period warefare.