It is actually a slightly oddity. Of course, its one of those oddities that exists for the simple reason that we're talking about a canon written in fits and starts over the course of decades by dozens if not hundreds of people. However, as time has gone on and Space Marines have become more and more special it has become a little odd that their tanks are STC tech.
So, bear with me here: as its always been explained a Standard Template Construct system was basically a massive fabricator that could create anything out of anything. Human colonies would set down on a world, the STC database would evaluate the resources at hand and provide the colony with the best version it of whatever it needed that could be created out of a series of standard schemata that it's AI could adapt to circumstances.
Nowadays, in the grim darkness of the even farther future, the STC patterns are revered as holy writ and there is a very real crossover between technological advancement and archaeology as tech-priests strive to discover lost STC data.
There are even whole bits of background about how the techno-archaeologist Arkhan Land found the STC schematics for what would become the Land Raider and Land Speeder. Further, there was a massive theological divide in the Adeptus Mechanicus when the Black Templars retrofitted linebreaker weapons onto the Land Raider as to whether it was heretical or not (read: a non-STC design or a forgotten STC design). The result of that debate basically boiled down to: “It works, therefore it must be compatible with a heretofore unknown STC or the Machine-God wouldn't allow it also half the chapters in the Imperium have adopted the design while we were debating and they're bigger than us.”.
The thing is, though, that Space Marines are the one place where non-STC tech was prevalent in the early Imperium.
The Primarch Project, the Astartes process, their power armour, their weapons tech and almost everything else was designed personally by the Emperor, the one being who could get around the Mechanicus because of their very convenient belief that he was the Machine-God personified. He was personally able to advance science and technology to his own design because his word on such matters was, effectively, considered itself to be an expression of the STC system.
So why are Land Raiders and Speeders STC designs? Why are Rhinos and all their variants based on the (Dark) Age Of Technology RH1N0 all-terrain vehicle?
Of course, but the I opened the post with the real answer and there's not really a way to reconcile it with modern canon. Though, there is a cheeky part of me that just wants to claim that the Emperor agrees with my complete disinterest in tanks and transport vehicles.
On the other hand, of course, being based on pre-Astartes STC sdesigns would explain why all Space Marine vehicles are actually slightly too small to accommodate the people they're meant to be transporting.