It's because they don't. It only happens once, like this thing of him thinking this fast. Or in my example above of him running at 13 trillion times speed of light. It doesn't get mentioned again later on, instead they just run at regular speed most of the time.

It's just that this attosecond thing has been posted so often by people that people who haven't read the comics think this does happen all the time and that the Flash is crazy OP and stupid when he gets hit by someone with human speed.

Most of the time in the comics the Flash is just a couple of thousand mph, sometimes when they really tap into the Speed Force for extra speed they go light speed but it's not like the Flash is constantly running at 13 trillion times the speed of light, that only happened once almost 15 years ago.

Those things get shown with other characters all the time, Superman, Batman do crazy "impossible" things but it's not as popular because their whole thing isn't just about one thing unlike the Flash who is all about speed.