There isn't a profound reason for this. It is just that in US customary units (don't call them "Imperial" any more - first, Britain no longer has an empire, and second, the UK no longer uses them in any serious engineering work) force, pressure, and stress are measured in weight units, but mass and density are measured in mass units.
There is a "weight unit of mass", the slug, but it is based on lengths measured in feet, not in inches, so it still needs another conversion factor of 12 applied to it for most engineering work.
It is generally less error-prone to include the conversion factor in the input to software as "the acceleration due to gravity" (or in Nastran the reciprocal of that number, for some perverse reason) rather than do all the conversions by hand. Unless you are working in the space industry (e.g. your are modelling a the deployment of a lunar lander) the value of the "magic conversion factor" is always the same.
The average engineer in the USA knows the density of mild steel is about 0.28lb/in^3, but probably wouldn't recognize 7.25e-4 as the density of anything!
Of course there is an analogous situation in SI units. If you want to include the weight of an object as a force in your model, the weight of a mass $m$ kilograms is $mg$ Newtons. But there are many engineering situations where the weight of an object is negligible compared with the other forces acting on it, and can be ignored.