If you were one of the incoming players you accumulated stuff fast, stuff that wasn't that easy to get rid of in a way that wasn't going to force you to lose progress. There's also a very low market selling limit. 20 per retainer doesn't even begin to handle it if you're trying to put stuff on to the market instead of just selling the vendors. This on top of the fact that your retainers are bringing back stuff, too.

Also, if you're a newer player trying to catch up one of the more efficient ways to do that is making sure you're getting double or triple credit for stuff by having several things started that could overlap. Relics have a lot of overlap potential. Right now the Yo-kai Watch event overlaps with ARR, HW, and ShB relics. Getting credit for multiple things simultaneously is a great way to reduce the feeling of grind.

There's also storage and market pressure if you're a crafter. The limited number of market spaces meaning you may need to sit on some stuff that you crafted is the market tanks suddenly or take a massive loss. There's also issues if the amount of something you want isn't available and instead you have 20 extra.

I find the bigger problem to be limited market selling. It's entirely possible they did that to keep the market stable, but I suspect its a performance consideration instead.

Right now the ratios are off where we're getting stuff faster than we can get rid of it barring taking, sometimes, large losses. We either need more market capacity, better storage, or reduced grind so that projects can finish faster. The current status quo isn't maintainable.