Oh, it's joining the system. It's having your financial activity monitored by the IRS (if just by proxy); the mere act of depositing the $10,000 will be reported thereto and documented for closer inspection (as in "how did a homeless guy suddenly come by a $10,000 deposit?") and require tax reporting (anything over $5,000 gets scrutinized for "drug violations"), which in turn will lead to documented addressing of quarterly payments (may be $0, but prove why). It's having your money held by people you don't trust behind an impersonal machine accessible only by a magic plastic card (which, under the circumstances, is almost as risky to have as the whole amount on hand in cash). Having the card, it's hard to not get sucked into using the card instead of cash (how many here experienced the abrupt realization we haven't carried cash for _months_?), leading to further data-mining by the system; not having instant/quick access to bank software, becomes harder to keep track of how much money one actually has on hand - risking expensive overdraws, which fire all kinds of additional attention from the system. You likely can't open an account without a fixed address (where to send official notices?); finding a sympathetic soul to provide one, now you're chained to that person, each at risk of consequences of the other doing something suspect. Who knows what other legal obligations he faces - the moment he opens the account, someone else may have immediate notification & right to seize the funds (tax delinquency, child support, etc). Further issues abound.
Easier just to have cash. Except that someplace like App Store isn't going to mail you an envelope of cash, they'll want a bank account to deposit in ... and now Leo is dealing with someone else holding his $10,000 and won't give it to him; that alone is enough to crush any faith in the system.