Many ISPs acquired IP addresses in the past for free. While they could sell them now on open market for $50/IP, it makes sense to hold them (as their price is expected to rise) and just use them for their customers (for fixed or monthly fee).

While technically they could rent them outside, such approach is not recommended as they would lose any control about how they are used, they can be used for spamming or hacking and can get on blacklists, which could damage their future selling price.

Presumably you're paying a monthly fee for your ISP, one which includes an IP of some sort. It doesn't cost anything for an ISP to give you a static address (which likely can be removed at their whim) rather than a dynamic address given you're likely to be needing an IP 24/7 (as you're the sort of person who knows what an IP address is)

If it costs $50 for a single IP, that would be 50 cents a month at a decent return on investment (and the value of that IP is still increasing), that is part of your monthly charge (which I suspect is far more than 50c).

Additionally, if you changed ISP you're going to lose that "investment" in your static IP, so you've basically paid $10 to lock yourself in.

I wonder how much IP address embezzlement goes on in big companies?

Might be the plot for a cyberpunk thriller. Not sure how that genre is doing, since one could argue that it's just real life now.