Why is Doom a tank, but not Mei?

Think about it. Mei has so much sustain already. Can change the map at will. Just make her model a little bit bigger, give her like “ice shield” or something and bam, perfect tank! Why make Doomfist a tank when you have a dps with a kit already built for a the role?

Sorry if this topic sucks, it has just been on my mind for a minute…

Why did you post this in the arcade collection forum. Googling “overwatch forums” gives this as the FIRST resutl.