Well, just because a mech was on the first exodus doesn't mean it survived the Pentagon Civil War, the second Exodus, and Klondike. Aren't we told that as headhunter mechs Exterminators were prime targets? And even after that there were still a few decades of Clan wars where a mech like an Exterminator might have been used (I can imagine a fair few being used by Clan Mongoose in units that hadn't yet received omnis, meaning they'd have been destroyed by the Jaguars). Honestly, the Clans just do so much fighting that they turn over mechs in a huge hurry.
The same of course holds true for any other mech. Sure, we know there were regiments upon regiments stashed away for rainy days, and I suppose its up to any given player what's in them. But I'd be surprised if any of the really really cool stuff didn't get fished out over the last two centuries.
