For this case of lateral PNP transistors, the reason is as Ken has said.
Because both the emitter and the collector are on the surface, making them both circular ensures that the distance between them, which is the same as the width of the base, is constant.
The properties of the bipolar transistors vary very strongly with the width of the base. If the width is not constant, then the current becomes crowded in only a part of the base and many characteristics become worse.
Unlike in lateral transistors, in vertical transistors the width of the base is not determined by geometry, but by doping doses and diffusion times, so the form of the emitter is less important.
Nonetheless, in early planar transistors the emitter was also circular. The reason is that in bipolar transistors with very narrow bases, the resistance of the narrow base layer becomes large and in the center of the base under the emitter, the base-emitter voltage drops to a lower value than at the terminals of the transistor, which makes the central part of the emitter and base non-functional (i.e. only a very small fraction of the current passes through there).
So in vertical transistors, only the periphery of the emitter matters. When it is circular, the symmetry guarantees that the current is uniformly distributed on the periphery, for maximum current capability.
Unfortunately, increasing the density of current per peripheral length of the emitter over a threshold triggers a positive feedback that will destroy the transistor if the current is not limited externally. This is usually the main factor that determines the specification of a maximum current for a bipolar transistor. If the current is non-uniform over the periphery, the threshold will be reached at a much lower current than computed by multiplying the threshold density with emitter perimeter.
Because there is a limit for amperes per millimeter of emitter periphery, to increase the maximum current in a given area, the form of the emitter must be changed from a circle to a form with a longer perimeter, without increasing the occupied area.
Early power transistors had various fancy forms for the emitters, e.g. Christmas tree, snow flake and so on. However, it was quite difficult to ensure that the current is distributed uniformly on the periphery of such complex forms.
Later, after the photolithography had improved and smaller dimensions were no longer problem, instead of having an emitter with a complex sinuous boundary, 2 simpler solutions have been adopted to increase the perimeter of the emitter. Either the transistor had a large number of small emitters connected in parallel, or it had one large emitter, but with a large number of small holes in the emitter (mesh emitter).