This discussion is off-topic like a fifteen-year-old is out of bounds.
Anyways, the reason why a soldier won't, for example, pick up and use a Tau rail gun is simple;
A) They have no clue how Tau rail guns work and the Mechanicus aren't likely to research it for them. Some IG engineers get put in prison for life for upgrading and improving their tanks without a Techpriest's permission - even if the changes make it better.
They have their own superstition about alien technology - who wouldn't after seeing a floating hover-tank?
C) They don't know how their own guns work, for a start. Even Space Marine's aren't sure how a bolter is constructed. They know it shoots things, and they can probably take it apart and put it back together, but they don't know the science of it.
That's where the idea of the machine spirit comes from - take care of the spirit through maintenance and regular cleaning and the machine will work for you. If you neglect it, by not repairing it when needed and 'honouring' the machine, then it'll break down when you need it most. Simple. It's all just natural logic flavoured by supersition. The Land Raider's Machine Spirit is actually a computer AI that takes over when the drivers are incapacitated.
You can pull comparisons to our world now, but keep this in mind; the Imperium is not like our world. It's incredibly religious, for one. Our world is considerably more free-thinking and liberal, at least at the moment. The Imperium has been effectively using the same gun for ten thousand years. There's been improvements, modifications and reformations, but technology doesn't advance so much as it stagnates. It's just how it is.
'Course, there's always the IG soldiers with pulse rifles who turned over to the Tau side, but that's a different kettle of fish.