The important thing with cartels et al. isn’t necessarily the easiest trafficking location as it is the best growing/production location, both from an agricultural and oversight point of view (think about the fact that the Medellín cartel was dominant in the US for some years despite being in Colombia).
The Sinaloa cartel and its affiliates can largely be tracked back to Pedro Pérez who got his start in the 60s. He grew enormous amount of marijuana in Sinaloa then allegedly came up with the idea of using planes to traffic it. That allowed him to focus his organization in a region where pot grew best without a huge regard to proximity to the border or a decent sea route.
His success created an entire new generation of successors who were predictably mostly located in the same region. At that point, some of the trafficking organizations were still using the methods that you describe, but the second Sinaloa generation was able to use their existing power to consolidate things and form the Guadalajara cartel.
This, somewhat predictably, didn’t create a lasting era of peace among traffickers, and those in Sinaloa soon became reliant upon routes through other areas, such as Tijuana. This created massive conflict, but ended up perhaps being the savior of Sinaloa. Because of the concentration of trafficking through Tijuana, it became the focus of both US and Mexican mitigation efforts. This seriously hindered the ability of the Tijuana organizations to continue their efforts in the manner they wanted, which had the side effect of somewhat freeing up Sinaloa from the massive conflicts that had arisen between them. Because of this, Sinaloa was largely able to sit back, produce the best/most product, and use their traditional aircraft means as a way to smuggle it. They’d even go so far as coming through Atlanta, which is hardly on the way.
So, to some extent, their lack of proximity to easy land/sea routes is what allowed them to prosper: they were in an area that could support the large-scale growth and production of marijuana, cocaine, and heroin while also having a novel means to smuggle it and avoiding the brunt of the government crackdowns that were focused on the areas that had the easiest access to traditional trafficking methods.
Edit: sources
Mexico and the US: Neighbors Confront Drug Trafficking