Are you suggesting all session ales are mediocre, or just the few you have tried? It would be beneficial to understand how one defines a session ale or beer.
Generally IME a session ale is one that is not defined or driven by intensity of flavours, rather how easy drinking it is, and refreshing, crisp finish. ABV is does not exceed 5%. There are countless excellent beers in many styles under 5%ABV.
I don't think brewers are intentionally brewing a mediocre product, they are brewing a beer, that is balancing flavour and style with a lighter alcohol content, that fills a space in their product offerings, and assumes there is a demand for the "session ale".
..but hey lets face it ....once you get "the taste" - every beer has the potential to become sessionable...hehe