99% uptime is 3.65 days (3 days 15hrs) of downtime.

99.9% uptime is 0.365 days (8 hrs 45 min) of downtime.

99.99% uptime is 0.0365 days (52 min 33 sec) of downtime.

It's also hard for the average person to catch unless they have users that are going to complain or a secondary service to constantly test the connection. And if they fail to deliver 99.99%, you just get a proportional credit. The benefits of advertising 99.99% usually outweigh the consequences of failing to deliver.