1) The passing environment 20 years ago was massively different. Particularly in terms of INTs, which are literally 30% less likely today, throwing more than Brady did in raw numbers is significantly worse.
2) Brady, pre-2007, wasn’t considered to be a particularly great passer – no one could have, or should have, foreseen him breaking yardage and TD records left and right, based on his performance during that time frame – aside from his exceptional “soft skills,” particularly decision-making. If you’re looking for someone to make a similar breakout, you’d look for similarity in that regard; Garoppolo isn’t even on the same planet there (he takes way more sacks, for another thing).
3) Any of Kittle, Samuel, Sanders, or probably any of the SF backs would instantly be the player at that position on those early-’00s Pats teams (maybe tied for the best, if you’re a Troy Brown enthusiast). And he especially didn’t have anyone remotely on Kittle’s level as a receiver until Moss showed up.
4) He’s about 3 years older than Brady was during that timeframe, i.e., closer to (realistically, currently at) his physical peak.
Garoppolo is a solid QB, maybe even an above average one. I’ll stick with the Kirk Cousins comparison. But he’s really not close to being on Brady’s level when he was starting out, and that was before Brady was Brady. More to the point, he’s a really different kind of player, more likely to try to hero-ball it, but (relatedly) much more mistake-prone.