The interest rates often make those unattractive. You also expose yourself to the risk of a margin call with its many implications. It isn’t something the average person should be doing casually.

IBKR will loan you vs your stocks at a cheaper rate than pretty much any place will for a heloc on your house. Not like a lot cheaper, but maybe 0.5%-1% cheaper if you have <<$1M, and a little better at higher amounts.

Most other brokers, even Schwab and Fidelity, will not.

Agree it's probably not a good idea for most people. (I might argue the same for a heloc, depending on what for, what emergency savings, what level of job security, etc)