Why do all vintage cameras look the same?

Look at the canon ae-1 the pentax me/me super, and the nikon fm3 a. They look almost exactly the same. The logo is in the same place and the model is too, same font as well, they're even the same texture! Was this some sort of marketing gimmick?

You mean the old mechanical 35mm film SLR's? That was during a time when function was valued much higher than form (and things were expected to work until the day you die). The shape was basically a tight fit around the light-tight box at the middle, film spools on both left and right sides and the pentaprism mirror in the hump on top. In a way, the shape worked. The left and right "wings" served as handles while the "hump" made it easier to "see" at eye-level.

Metal was the material of choice since plastics were basically at an infantile stage yet and leather was the standard covering (from sofas to car tops). Font? They differed if you really look "deeply" into it. The placing of the brand and model? It wasn't standard if you have already seen a lot of cameras. Gimmick? Nah! Just like today, they did it with those gorgeous models doing the demos.

All the SLR film cameras had MIRRORS inside their manual focusing ( then AF made it all electronic) bodies. Not exactly a market franchise trick----most had shutter speed differences ( up to 1/8000 like in SLRs of Nikon and Canon ), lots of AF sensors for the AF bodies, real magnesium frames, if not, titanium! It was a CREATIVE era in FILM SLRs---the bottom line---they deployed mirrors so the way the interiors were assembled had to conform to the mechanisms and some tech gizmo applications to the camera.

It was NO gimmick.

It was more likely an attempt to standardize the look and feel of those cameras.

The pro cameras were still more robust and a bit heavier as are the new digital SLR cameras of today. Leather or "leather feel" covering was also standard

That classic design was the optimum one for that type of camera. They all had to accommodate a mirror, shutter and electronics and so adopted similar designs.
Same as DSLRs now.
Some manufacturers did try to change the style but they never caught on… People wanted the usual SLR shape.