> I have no idea why motorcycles aren't more used, especially in areas where there's no real winter.
As a motorcycle rider myself, I think you simply don't understand how bad it gets in ATL (both during summer and winter).
I currently live in Seattle, and it rains quite often, but it is just a drizzle most of the time, and I can still ride quite comfortably. Summer is a bit too hot at times, but nothing wild. Winters are fine too, aside from those very rare snow days.
Before moving to Seattle, I used to live in ATL, and I can tell you I would never ride a motorcycle there for multiple reasons.
1. It doesn't rain as often in ATL as it does in Seattle, but when it does, it comes out of nowhere and it pours buckets. Impossible to ride, even driving a car feels extremely risky with those rains.
2. Winters in ATL tend to be colder than in Seattle a lot of times (not averaged out, but on a day-to-day basis; e.g., one day in January you wear a winter coat, a week after you wear shorts and a tshirt, and another week later you are back to your winter coat), and summers are always much much hotter and much more humid. Weather swings much stronger in both directions. Humidity is a total killer. Riding in ATL during summer is pretty much impossible, unless you invest into cooling solutions (like cooling vests and such, which barely work for longer than 20 minutes at a time, as I found out from my own experience). You mention Phoenix, but the crucial difference is that air in Phoenix is very dry, which is much better for riding (or being outside in general). I witnessed 95F+ temps in Seattle (which also has dry air) a few times, and didn't even blink an eye and was able to stay comfortably outside. In ATL, I couldn't stand being outside even for a few mins once the temps climbed above 75F. Overall, I would say ATL has way way less rideable days than Seattle, and that's despite the Seattle rain.
3. Last, but not the least, drivers. While this isn't the main factor in my decision to never ride in ATL, it could be for a lot of people. Drivers are very aggressive, with many coming from very rural or suburban GA, driving giant trucks and having some animosity towards motorcycle riders specifically. I am not joking about the last one, I legitimately heard multiple people claim they wished it was legal to hit motorcycle riders on the road. But again, this point is just a minor one for me, as it is much more subjective than the first two and can be just written off as my personal experience aka anecdata.
With that in mind, I just want to say that this is not my critique of ATL as a city overall, just when it comes to riding a motorcycle there. If ATL manages to get the career opportunities for devs going with all those tech companies opening big offices there, and it all succeeds, I would consider moving back, despite the weather. ATL has quite a lot of things worthy of praise and appreciation that I miss while living in Seattle.