>>282948373
Because in some digimon's cases it an appealing apperance is a powerful tool such as the case of Venusmon or MarineAngemon, in other cases its an expression of the type of data or environment they've taken in and adapted to as Lilithmon holds the sin code of Lust and is derived from data of the demon Lilith.
>>282948481
Digimon do not reproduce, more digimon are brought into being as the digital world expands. The means by which it expands aren't explained or given much detail since it isn't important beyond explaining how new species can come into being, nobody is coding new digimon into existence not even the host computer(s) or a higher being. There can be a limit to the rate of expansion or space within a digital world as shown by the X-Antibody storyline.