Why do people always refer to tourney players and arbitrary tier list like that's the be all end all. None of that ever matters in pub play. The majority of regular everyday players don't know about the meta. Only defensive character that's situational is trobborjn. Everyone is viable. Mei and Junkrat are good on offense and defense. Even widowmaker and Hanzo(most ppl just suck wit them) Just cause some Asian pro doesn't play them doesn't mean they aren't viable. And the Mercy slander is idiotic
Answer: After playing a 15-30 minutes game, people like to win at the end.
Mercy has a pretty poor kit... and is outclassed by other supports. There are sites that aggregate gameplay data across players and among the supports she has the worst win % and in the bottom 6 across all characters. I know you probably think it's all bs, so you can ignore that and I'll talk through why I think she's the weakest healer support.
Mercy's ultimate is her only redeeming factor, the rest of her kit is pretty lackluster given that you sacrifice while supports don't have to and do just as good if not a better job. She has a single target heal or a 30% damage boost where she cannot do anything else while it's happening or it's interrupted. Her heal/damage boost has a handy beam which lets the enemy team know your location and she has no way to defend herself, by the time you swap to the pistol you're usually dead and your only method of survival is to float away. She lacks the utility other supports bring to the table and what she does do isn't all that impressive. If you look at it she really only has one ability (heal/dps buff) to help her team, compared to her counterparts that can do alot more and heal. She's useless by alone and can barely attack.
Now look at Zenyetta for instance, he has a single target heal yet he is able to attack, debuff and do other abilities while it's healing, assuming they're not too far away and still in sight. He can debuff which results in 30% increased damage for everyone that attacks that debuffed target, basically making tanks far less scary. He can be healing, debuff and do a ton of damage all at the same time.
Lucio, has a area of effect skills that you can alternate between healing and speed boost. While these is occurring he can continue to shoot and use abilities without interrupting the heal/speed boost. The speed boost is pretty nice, in that it adds your team evasion and helps everyone move faster. He can boop people off ledges, wallride, kill stragglers and hold objectives.
Ana, can fire her rifle to heal and infact it can heal at a faster rate than Mercy's heal. The caveat there is there's some skillcap in that you need to aim. She has a sleep dart to take out scary targets. She has a grenade that heals in a small area and debuffs enemies so that they cannot heal briefly. She can help open up opportunities to break into a defensive position or prevent something catastrophic by sleeping someone
I can't really speak on Symmetra as I haven't played her much since the change. She's the odd ball in the bunch since she doesn't heal. But statistically she's in a very good place, with one of the highest win % across all characters and people crying on the OW forums that she should be nerfed.
