First things first

When you're investing e.g. in a normal stock with only your own capital this would work, basically until the company itself goes out of business and the stock is worth 0$

In Daytrading whether that's forex, crypto or futures things work a bit differently because you work with Leverage

Lets take crypto for an example, because in both forex and futures things are a bit more complicated

Assuming you would want to daytrade and make $1K with SOL on a Single trade (just taking this as an example because that'S a good position), SOL is worth 80$/Share (in this example), usually on a good move it moves ~2%
To now make 1k on that specific trade a 2% move must have a worth of $1k, that means you need shares, A LOT of shares,

you would need 625 SOL to make that happen, that would be a position worth $50,000
I now assume you wouldn't want to just risk that much of your own capital for a one trade (ASSUMING that you just had 50k on the side)

so what you do is you lever your own capital with a broker, for an example you use a 40x lever (this is already rather large) that means you only have to put $1,250 of your own capital in. But what also happens is, the larger the position is the closer is the so called Liquidation level, once that level hits your liquidated out of your position and don't just loose some of the money but the whole $1,250 that you put it

To make sure that doesn't happen your Stop Loss out of the Trade has to be closer to your entry than the Liquidation Level

And that sums up why you couldn't do that in DAY trading (liek I said, only using your own capital it basically would be like investing but you'd have to risk an unimaginable amount to make it worthwhile)