It is widely believed one of the largest constituents of WD-40 is fish oil - which is the reason why the spray is water-repelling, and leaves a protective coating. As a lubricant, it is on the lower levels of lubricating ability.
There are better seat slide track products available. Look for some of the door lock spray lubricants, that leave a lubricant coating.
My personal choice is the spray lubricants that contain PTFE and Molybdenum Disulphide. MoS2 is well known for its dry lubricating abilities.
Greases are constructed from heavy-consistency metallic soap fillers, that contain mineral oils suspended in the filler material.
Over time (a month or two) the oils will drain away from the metallic soap filler, which then can no longer carry out its job of lubrication.