that doesn't make sense for you maybe, but that's not the point. It doesn't matter what you think what should be right. It's how the system works, no matter how unfair you think it is.
Cash is a pain to move around, and expensive to handle, banks and drug dealers know this first hand. Hence drug kings need to use cryptos
They will ask you first how you got the money to invest, and that can be a problem for some. The point is that if the bank or the regulator has any sense you are not legit, they will block you with unreasonable requests. Then you need to hire an expensive tax lawyers and get the funds release. Make perfect sense for millions, but if you get blocked for $100K or less, which happens more often than you think, then it's quickly an expensive nightmare.
And good luck detailing all your trades from different exchanges in something easy to understand for an IRS officer, more likely they won't bother checking if you are doing 100s of trades per month, and instead will tax you on the whole proceeds, which could be 100% of your money if they do it right