This is quite interesting question my friend. This kind of question only comes in mind when we start to think why ? How ? and try to understand each and everything. I don't know if my answer will help you that much but I read somewhere about this long ago and I also have some idea so let me try.

Let's talk about lens first that why camera lenses are round only ? why not other shape ? other shapes are not possible ?

First of all Camera lens alone don't produces the picture, other key elements also take part in it. In older camera it was "FILM" and in modern camera it's image sensor. Light bounces from the object (that we are capturing) to the camera lens - the main function of lens is to bend the light and focuses on "FILM" or "Image Sensor" that way that it help to produce the best quality image. Now after the so many experiments experts and photographers come to same point that Round Lens do this job better instead of other shape. So all lenses comes with round shape. Other shapes are also possible and some companies are working on it too so they can come up with the new solution.

Now next question pop ups that if they found out that Round Lens can do better job then why they were using rectangular film and why not round. There were many reasons behind that some where technical some where with common sense. Important reason was when someone draws the painting or the posters what shapes are they ? In which shape it looks great ? everyone usually displays painting , photos and all things on walls ? & walls shapes are - rectangular or square. Another thing is it's easy to create rectangular or square frames.

Hope this answer is helpful.

Once again - you asked the good question my friend @Xb7xavi.