Well, not always 200 years actually. Not even close.
Arena was set in 3E 399.
Daggerfall was set in 3E 405 - 6 years after Arena. (Daggerfall also officially ended 12 years later, in 417).
Morrowind was in 3E 427 - 22 years after Daggerfall started.
Oblivion in 3E 433 - 6 years after Morrowind.
Skyrim broke the trend by being 201 years after Oblivion.
And then you have Redguard and Elder Scrolls Online. Both being set in 2E, about 400 years before Arena. And there is a lot of debate about when Battlespire was set, so I won't even try and explain that one. But the vast majority of games (especially as some do not consider ESO to be canon - I think they are wrong, it is canon, like it or not) are clumped close together.