There doesn't need to be a "why?", we just need an absence of a "why not?".

Something as simple as the game of life shows you how highly complex emergent behaviour can emerge from incredibly simple rules.

The relevant molecules are made of very simple pieces that like to stick to each other and the way they stick influences their neighbors. It's very feasible to stumble into a pattern that spreads, and from there all you need is time and luck for those patterns to mutate into better spreaders, often getting more complicated as they do so in competition with other patterns.

These are natural outcomes of evolution, you see the same things pop up very easily with simulated evolution* of even non-organic structures.

* that is, make a design (by any method including literally randomly), replicate it imperfectly m times, sort by "best" according to some fitness function (which for us is something we like, for nature it's just survival to reproductive age), pick best n, mix and match, repeat