The buld in flash is not very expensive. I remeber that one could get 1 way cameras with a flash that where not that expensive. External flashes add complications to the control and there are different types. Though using the same hot shoe, some of the older ones can be quite nasty, with a relatively high voltage at the contracts. The internal flash also does not need to enter the flash strength as an extra parameter. An external flash may be more powerfull, but for most uses the internal one is good enough, especially with moder relatively fast film.
After I got my first digital camera in 2000, I actually started taking more pictures with the conventional one than before. One kind of got used to taking more pictures and the first digital cameras were a bit limited with high contrast. Most pictures were still quite good, even with only 2 Mpixels.