Question

why 293 T cells are only used for virus production

Answer 1

It might differ in terms of efficiency of virus production.
The HEK293T cells harbour the large T antigen. This allows episomal DNA replication from SV40 ORIs. One could argue because of the extra plasmid amplification within cells which could get more virus and the the transfection should be more stable.
On the other hand I assume to transfect the cells and collect virus only few days thereafter. HEK293 cells are incredibly easy to transfect so after the transfection the cells will produce virus at their limits anyway.
Besides the possible differences in efficiency I can assure that also HEK cells without large T can produce virus. We can also use HEK293 cells instead of HEK293T cells. We should provide the cells all they need for virus production with the plasmids