Time saved in procuring said grounds, making the coffee, cleaning the press, increase in variety and availability (many locations) and likely increase in quality depending on your skills/preference.

No need to carry a french press and coffee grounds (and hot water source) when you wander outside. Yes, coffee is cheaper if you make it at home, but people still go to coffee shops.

People don't want to take the time/resources to invest in making it themselves. Simple as that. Some people are fine (stupid?) with paying for the convenience.