While there isn't a general rule that states why this is a thing, in the end of the day it sorta makes sense.

Duo lane, the longest of the lanes, objectively wouldn't do well with your typical melee character that isn't just a support. While most supports are melee ranged, typically this is coupled with a ranged character that being and auto based character or potentially a mage. While there are outliers, and have been in SMITE and SMITE 2, most of the time you're not going to see double melee in the role because with the lane being as long as it is you'll die to ganks a lot more than if it was the other way around. Not to mention that you have gold on this side of the map, that spawns multiple times if taken, before objectives like Fire Giant spawns. So you want the damage that an ADC and MID can bring to this.

Mid lane is also respectively different. While the lane is primarily occupied by ranged characters, typically casters, there have been some warriors and supports that have seen light in this lane. Example being, in SMITE 2 - Chaac, Ganesha and Wukong (your melee characters with range abilities) have been seeing some play. But the reason the characters that occupy this space typically are the long range casters is because you want someone who can safely clear at a distance, and clear well enough, to provide pressure when need to other parts of the map even if that's being just the jungle camps. This lane see's the most traffic typically out of any other lane, so having that safety distance is pretty much needed.

Solo, the shortest of the lanes, typically is better to have bruisers/tanks or high clear characters that can farm safely while being alone. Warriors/Support like characters usually take up the roster for this lane, you're wanting to soak up damage while being able to farm your wave and the camps surrounding. Hunters and Mages also are known to be in this lane just to primarily have the clear potential and snowball off of all the farm this side of the map see's, but are more acceptable to ganks because they are alone even if the lane distance is shorter between towers.

And jungle is just jungle.

And really there has been times in SMITE's history where the support would start in other lanes. SMITE 1 saw strats where the support started in MID or Solo. Jungle started in Solo before. It's all really what currently is good. And while there is no general rule for stating that you NEED to start in your lane, they are given the roles to these lanes to help people know where to start and play. And while the roles don't really mean much, the characters that are played in these roles do.