In this case, it's because it's a reboot. Calling MW (2019) "MW4" would've been arguably more confusing because the new and old games are completely unrelated to each other. Totally different stories, part of a single timeline built off of the Black Ops universe, and a reset of fan favorite characters that have been brought back. There are already a surprising amount of people who don't understand that, I've seen people shocked to find out it's not just a prequel and that new Ghost is likely to stay alive permanently for those big bucks.

Besides, it's not like the numbers used to properly correlate, either. Despite Doom 64, there aren't 63 other Doom games, and Doom 3 is a Doom (1993) prequel. That kind of craziness happened way before DOOM (2016).