Most rental agreements strictly forbid this. I can't compare irregular activities to irregular activities because you can always escalate it to some nonsense money making action favoring one side over another.

But supposing your agreement explicitly allows sub let's without landlord arbitrary approval, the owner is still coming out on top because the activity is simply reducing their monthly outlay while still building equity, the renter is only reducing outlay, but still has zero equity at the end of the day.

More importantly, my mortgage payment will stay the same, fixed in dollars at the year I borrowed the money (in my case 2006 dollars). While rents will continue to increase. For example, at some point in the future, I'll theoretically even be able to rent a single bedroom out that will cover my entire mortgage payment simply because of this simple truth. I'll live "for free". But your base rent price will continue increasing such that you're monthly burden will stay a significant part of your monthly income. Even if you pass along as much as you can to a subletter, that percentage won't change much while mine will continuously reduce.