the difference is that Mario 64 has satisfying controls and movement options.
Astro Bot has basically nothing except decent technical polish and fan service.
it's such an insanely shallow game.
Astro Bot movement: Run, Jump, Hover, Punch, Spinattack.
Mario 64 movement: Run, Jump, Double/Triple Jump, Crouch, Punch, Jump Kick, Slide Kick, Crouch Kick, Backflip, Sideflip, Dive Jump, Wall Kick... and that's just the basic options, you can use all of these in more advanced ways, like the Jump Kick can function as a double jump in some situations or as a way to keep your momentum out of a dive etc.
the only advanced tech in Astro Bot is that attacking mid air sliiiiiiighly extends your jump... basically a less effective version of New SMB's mid-air twirl
in short, you can get better at Mario 64 as you play it longer, but Astro Bot's skill ceiling is so low that by the end of the first 4 levels or so you already mastered nearly everything there is to master. and in combination with the the level design it doesn't really allow for player expression either.
finding the secrets is also not a challenge, because the levels are mostly linear with very little exploration outside of a handful of exceptions.
the biggest challenge when trying to find secrets is the unbelievably slow camera turn speed, which is so slow that you'd be excused to think your right stick is broken.
and no, a platformer doesn't necessarily need to be hard,
nor does it absolutely need to be mechanically deep or super innovative...
but it should be one of these at the very least, or else it has very little going for it other than "huh, that looks cute".
Super Meat Boy isn't innovative or mechanically deep, but it's challenging.
Super Mario Galaxy isn't that mechanically deep but somewhat challenging and innovative.
Super Mario Odyssey is mechanically deep and innovative, but has almost no challenge outside of the last bonus worlds.
Super Mario 64 is mechanically deep, was innovative at the time, but isn't that challenging.
Penny’s Big Breakaway is mechanically deep, somewhat innovative, and somewhat challenging.
Super Mario Bros. was innovative at the time, not really deep, but challenging.
Yellow Taxi Goes Vroom is mechanically not that deep but has some advanced tech to learn and master, it's innovative and somewhat challenging.
Donkey Kong Country is mechanically not that deep (some advanced tech but not much), not really innovative, but it's challenging.
Shovel Knight has some depth, isn't really innovative, but is somewhat challenging.
Shovel Knight: Plague of Shadows has depth, is innovative and is somewhat challenging.
and then we have Astro Bot, which mechanically has no depth, which has zero innovation, and no challenge outside of 2 or 3 bonus levels.
it just stands out to anyone who dabbles in this genre imo. everything it does has been done before, after a few levels you basically mastered it completely already, and if you played maybe half of the games I listed above, you should at no point feel any challenge playing it.