My personal theory is that the Republic borrowed designs from the Mandalorians during the Mandalorian Wars. We know the Republic captured and studied Mandalorian armour designs during the war. This was stated in one of the Kotor comic issues.
But why take the enemy's armour design? Simple. Mandalorians wore armour that was far superior to Republic armour during the wars. That's one of the reasons they were so formidable. So instead of designing completely "Republic armour" for their soldiers, they could borrow enemy technology and use it against them. Perhaps an early version of the "Clone Trooper style" armours were present in the later days of the Mandalorian Wars after Revan took over the military. He used Mandalore's own tactics against him, after all. Not a far reach to use Mandalore's own technology against him.
Then, centuries later, the armour proves to be very effective, and the Republic mass produces it for their troops. Millennia later, the Kaminoans look to both Jango Fett and the early Republic soldiers for inspiration in outfitting the new Clone Army. Again, why design armour from the ground up if history has proven a design to be very effective?
That's how I see it and how I justify it. Doesn't break the immersion for me, but I admit I did worry about it when we first got a glimpse of the Trooper class 4 years ago.
Cars still have the same basic designs as they did in the early days in the real world, 100 years later. Computers haven't changed all that much since the 70's either. Just some real life examples to accompany this (very educational) rant of mine.