This is one of those aspects of the game that makes less sense looking back than it did at the time.
Originally, LosTech meant working or restorable military equipment or civilian infrastructural items that could be put back into use or repaired and made functional. A cache might have anything from tanks with full-armour and functional weapons to LAMs, but it wouldn't have an ER Large Laser, because that didn't exist yet in the rules.
The fluff made allusions to superior star-league-era equipment, but didn't give you a playable example of it. Some passages used the decline of technology to explain away bothersome elements of the game itself; mainly the abominably-short-ranges the game worked with.
Even a pristine factory-to-cache Marauder couldn't act like an SLDF mech, because age had caused the computers to degrade.
In retrospect; once products like TRO:2750 came out, it began to seem weird that no-one ever found a cache with a working Gauss Rifle or ERPPC in it.
But after the earliest scenario packs, you started to see examples of the first operational recovered LosTech, because common-sense was eroding the GrimDark Mad-Max-ness of the original works, resulting in "Freezers". Even then, you wouldn't find "Freezers" in a cache because they were a new-fangled retrotech bootstrapping of DHS technology.
For a long time, there was a disconnect between the fluff (Novels) and the sourcebooks in terms of what was and was not available when and in what quantities. More recent works have done a lot to rationalize this.
But we're still stuck with this perspective that makes a core aspect of the early game seem different based on what angle you come at it from.
BT:PC made things a lot worse. While it's arguable how canon MWO is and when it takes place and how the timeline unfolds...from early on, the storyline of BT:PC is supposed to be largely canon and it pivots on massive quantities of advanced tech being available to the player fairly easily and far earlier than it should be. So, if you come to the tabletop game from these two later games, the way most players engage with the earlier or classic eras seems skewed.
If I was to run a classic-era campaign now, set in the late 3rd-4SW era, I'd have a *very* hard time realistically insisting that it was entirely impossible to come across real LosTech in a LosTech cache. At the point I, as a GM am saying that you, the player, who has beat all the rolls to actually find a LosTech-whatever in the cache, are stuck with a varied-degree of lemon you have to replace some/all of the gear on to make it work, I'm just bending over backwards to keep you from having a treat.
Opinions will vary; some might say: "The advanced systems just do not work" and you have to replace them, others that they do, but they can never be replaced, repaired or replenished. There is no way I could give myself a pass on making a player replace every piece of advanced equipment on their dusty new toy, it's just not fun.
What I would do is make it a maintenance hog, with items that are nearly impossible to repair and that essentially cannot be replaced once they are gone. In some cases that might include ammo too. But if a player or their party actually manage to: A) Find a cache. B) The cache has potentially useful stuff in it that has not rotted out. C) They can recover *something* of the cache. D) Something they recover is lostech and finally: E) it works...Do I really need to be that stuck on them having an Ultra-AC/5? Or even a Gauss Rifle? No.
But why?
Because later canon explicitly allows for all of this. There is the TBH(IIRC) story of a farm using an heirloom lostech ECM suite to spoof some Kuritans, MW: By Blood Betrayed (novel) describing Hopper Morrison re-equipping most of a pirate band with SLDF mechs recovered from a cache and even TRO: 3039 has the fluff for the Hussar giving a functional ER Large Laser to some bandit waster on Astrokazy who acts as an enforcer for one of the local petty nobles.
If it's *my* game, I can just ignore all that, but frankly I feel I lose credibility with myself when I do things like that.
We almost had an alternative with how the early fluff for the Dark Age was written, with that era approaching a "MadMax-Redux" feel with Clantech in the mix. But later works have moved away from the effects of disarmament causing what amounts to a military tech-collapse.