FIFA has been thinking about it for long, but that one is actually difficult.
Because actual playing time is around 55 minutes per game. Footballers right now can't play 90 minutes effective playing time, and fans would hate it if the game is lasting beyond 3 hours for the sake of it.
So to do it, you have to change football match into 60 minutes, or less, games. And while this looks good, as it is essentially the sane, it will be drastic change to the game. 90 minutes is a football tradition that is hard wired at fans mind atm. So, you can see the hesitation is warranted.
Also, 55 minutes game is weird, 60 minutes game adds another mileage on players, who are already complaining. Never mind airing time implications.
They tried the long added times in the WC, and tbh I enjoyed it, but it seems it was unpopular as they didn't continue with it.