For me it's a very simple matter: If they can just train in their lower forms, why to transformations even exist? in and out of universe this makes little sense. A while ago I did some digging, and Toriyama actually seems to share this perspective with me:
Incidentally, what led you to have Goku become a Super Saiyan, or the villains to level up?
I was feeling that there were limits on mere strength, so I was generally always struggling to come up with something.
And thinking about it, the story seems to support this notion as well. Vegeta says on Namek there is no warrior that can be as powerful as a Super Saiyan, and later admits he didn't even want to become a Super Saiyan, but had to in other to compete with Goku:
Chapter: 280 (DBZ 86), P3.1-2
Vegeta: “A Super Saiyan appears once every thousand years… a Saiyan who overcomes the wall which no warrior, no matter how gifted, can overcome…That’s supposed to just be a stupid tradition…And even if the legend were true…only I would have the potential to become a Super Saiyan.”
Chapter 343 (DBZ 149), P11.1-5
Kuririn: “Th-that’s impossible…! Wh-why can he become a Super Saiyan…?! Don’t you have to have a tranquil heart to become one…!?”
Vegeta: “I was tranquil…Tranquil and pure…Pure evil, that is…I wished to get strong just by training earnestly…And so I went through stupendous training over and over again…Eventually, I realized my limits…Through my anger towards myself, I suddenly awakened…into a Super Saiyan!”
Guides like Daizenshuu 7, SEG and EML to describe something similar about how Super Saiyan surpasses a Saiyan's natural limit or how Vegeta became a SSJ upon hitting his own limits. By the Boo Arc we even have Goku being impressed by Kaioshin being able of one shoting Freeza and Beerus flat out saying Goku can't beat Freeza without SSJ years later, so Saiyans aren't supposed to be that impressive without their transformations.