What Liam said. The Age of War and Star League eras are not the Succession Wars, where all available mechs are maintained with salvage so much that they can last for centuries with the proper care. Those bygone days are more like the modern world, where old and obsolete units are retired, scrapped, or even recycled for the metal. The earliest generations of mechs likely only served a few decades before being parted out or melted down, with the few survivors being those that find their way into private collections or became museum pieces/gate guardians.
Later builds would have served and then been shuffled off to Terran militias, which bore the brunt of Amaris's initial takeover of the Hegemony. Those Mackies would have either fallen in battle during those first days of the war, or been abandoned(Mackies can fill many roles, but guerrilla fighter isn't one of them), put into service by the AEAF, and subsequently destroyed by the SLDF.
Which is the Western Model . . .
Frontline service with elite units for designed purpose (European units during the Cold War, Korea DMZ in Cold War, N Atlantic fleets, etc)
Production gets around to filling all regular commands to become the standard
Package upgrades to weapon system- better guns, engines, sensors, armor, etc
Pulled from elite units to be replaced with the latest thing, hands off to the better reserve units- starts selling to allies, or in this case the Houses
Design gets repurposed for specialization in elite/frontline service (Wild Weasel Phantoms, EF-111 EW platforms, WWII diesel subs for R&D- Archerfish, M-60s as bridgelayers/mineclearing/CEV) this step sometimes combines with the previous one- also becomes training vehicle
Fills reserve units and is in limited regular units, oldest examples start getting mothballed or scrapped- or ones that have structural wear/faults
No longer in regular units, better reserve units start replacing with 'latest' thing
Phased out of reserve units into mothballs- gets used for targets- beyond strategic minimum reserve, gets scrapped
Soviet model?
Never throw it away, it can be used somewhere!
So yeah . . . Hegemony militia was still using the Mackie up to Aramis per the fluff, but not the original model since it had kept being upgraded. Probably had depots set up around the Hegemony like what Lima was in the 50s and Davis-Mothman is for the USAF today. New Dallas's underground storage facility was just that sort of stockpile.
Since IMO they would be regional, the question is could someone have moved or hidden the mechs, armor, ASF, small craft and maybe even DS before Aramis folks laid hands on them. We know in the case of what was found on Helm, that could very well have happened in other place.