Why aren't grid fins used as wings for low speed aircraft?
No, not as/on aircraft, and there are good reasons.  They're useful elsewhere, and should make good kites.
Prototype multiplanes flew, quadruplanes reached production, triplanes served operationally in WWI, and biplanes were retired from almost all uses (military and civilian) by early WWII. The biplanes that still fly for purposes other than display are used for low-speed work.
As speeds increase - and we almost always want to get people and goods somewhere quickly - the power lost to drag increases dramatically, in proportion to the cube of the airspeed.  Any construction that increases drag is going to limit speed, payload, range or probably all of them for a given engine power. Biplanes (and to an even greater extent multiplanes) are inefficient at lifting because of the interference between the airflow over the lower wing and under the upper - now squeeze the wings closer together so you can fit more, and this gets worse.
The surfaces (or struts/wires) perpendicular to the lifting surfaces, making up the grid fin or the bracing of a multiplane don't contribute lift but do contribute drag.
Grid fins are sometimes useful: they're compact, more so if the aspect ratio means they can be stowed as by SpaceX and on missiles/glide bombs.  SpaceX use the grid fins in landing, where drag isn't a downside so long as it doesn't cause too much heating.