Deposit APY for stablecoins

Can someone explain on a high level why the deposit/supply APYs for DEFI lending platforms are so wildly different for different stablecoins? For example, right now, on Compound Dai/USDC/USDT have 3.57/7.9/5.97%, respectively, which is a lot of variance.

Usually I see the same on AAVE too, but exactly today the rates are pretty close to each other (GUSD being the exception, which is not on Compound, having a whopping 23.2%).

And this is not limited to Ethereum based DEFI, I see the same on other chains too (like Geist Finance etc).

Note1: I am not asking about comparing these platforms, but more about asking why within a given platform there is so much variance between the rates of supposedly same/similar assets.

Note2: I understand that this is all about supply/demand for the given coin/app (e.g. supply/demand for DAI on Compound specifically), but what's not clear to me is why is there no "automatic arbitration" happening here? I would expect people with stablecoin X earning half the rate than stablecoin Y would just switch to stablecoin Y to earn more, but this is clearly not happening. Sure, I understand that not all stablecoins are equal (USDT, khm...), but really, is there so much difference between DAI and USDC, for example, that justifies a 2x rate for latter? (and I understand that there are different types of stablecoins - centralised, colletarelized and algorithmic - but I don't think an "average Joe" cares).

Note3: I am talking about DEFI lending platforms, not the centralized ones (Celsius, BlockFi, Nexo etc). Those are almost always have the same rate for the different stablecoins, not just within the given app, but even across apps (which makes sense, CEFI market competition and all).