Because Hanzo fulfills the role of a sniper,
BBBUT WHAT IZ A SNIPER?
If you want a text book definition of a sniper in overwatch its this:
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An offensive oriented character that deals 200 hp damage or higher to a single target through one precise critical shot without any intervention and from a range
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Sniper its not an official category but that doesnt mean it doesnt hold any meaning (similar to how offtank and dive arent official categories either but they mean something to the overwatch population); for official I mean easily recognizable in the selection screen where as of currently theres only 3 categories -> offense, tanks and supports.
BBUT ANA IZ A SNIPER!
By the previous definition Ana is NOT a sniper, regardless of what lore says Ingame Ana only has high precision shots but she cant crit and her goal isnt to do damage, she is not played like a traditional sniper and she shouldnt be considered one, remember snipers are offense oriented characters that are able to deal 200 hp damage or higher to a single target through one precise critical shot without any intervention and from a range
Ana is just a support character with a highly precise weapon. She’s supposed to be versatile. She has the scope and hitscan (when zoomed) so she can heal at range, but she isn’t a replacement for a widow/hanzo. All things being equal an Anna going against a widow should pretty much always lose. Remember that she can’t headshot, that should tell you a lot about how she’s intended to be played. Long range chip damage is doable, but she’s a healing support character first and foremost. Not to mention the huge amount of time she spends shooting from hipfire.
BBUT ASHE IZ A SNIPER!
By the previous definition Ashe is NOT a sniper, close but no cigar… even though her kit borderlines on what a sniper is capable of her falloff and unassisted dmg output is not quite up there when compared to the other more capable and established snipers (Hanzo and Widow).
BBUT THE TOO MANY SNIPERS WARNING!
Yeah, there used to be a warning for too many builders as well, that prompt has proven to be heavily outdated and irrelevant in the long run and shouldn’t be taken seriously.
The “too many snipers” selection screen warning message was implemented years ago from the beta with the sole purpose to hint new players to be aware that the team composition might not be well rounded But you can ignore this and still win, players should take it for what it is, a mere suggestion. In that regard ana contributes towards the “too many snipers” warning because she requires high precision shots and is used at a distance like hanzo and widow but warns you that using many heroes like that can result in pathetic matches where theres not enough bodies in the objective.
I hope thats enough to clarify things *drops mic / thread