Always someone saying absolutely nothing of value. Thanks for being that someone?
It's mainly because Quack is used to the wargaming scene and know there are a few flaws in your statement according to meta and how games are usually played out.
If the resto druid is playing mid, without carrying the flag, there is still no guarantee that you will win mid. You might have more healing in mid, but the damage is close to equal on both sides, assuming the resto would throw a few dots when someone comes in range for just that little extra, you will be ahead by a small margin.
In this scenario, it basically comes down to who can land the best coordinated kills and snowball off that. It's simply not worth the risk to have resto druid in mid fight without a pick, because if you do get pushed back slightly, then that's a
huuuuuuuuge mistake, because that opens up for the opposing team to go for a super easy cap, especially since you don't have pick, not to mention that simply having a pick without enemy team having a pick means that there is way more pressure, because the team that doesn't have pick now can't afford to fuck up in either mid or objectively since their FC is out of stealth in mid, else it simply results in a cap for the team that has pick.
It's basically more pressure to simply pick the flag for your team, connect with them in mid,
and then start healing your team until stacks hit. It's much, much less risky, and even then, the druid still needs to keep track of people going wide, whether the rogue (if they have one) and/or boomkin is stealthed etc. because during those times you should be in bear (because it means that they are going for an opener), which means you won't be healing at those moments. Also, when stacks hit 4/5/6 (depending on how you feel it out) you basically sit bear all the time with a few exceptions (such as when rogue is dead / not in stealth - same for boomy, and you know for a fact that you won't get jumped). During this time where you sit bear, you obviously don't contribute to heals either.
If you heal in mid as resto druid without pick, you're putting all of your eggs in one basket. You then
have to win mid, and in a very mid oriented meta, that's just way too risky, because it's not really a guaranteed win in mid.