Our planet saw a serious period of glaciation so many irregular features were revealed only recently. Ice isn't really very fluid so it creates irregularities, water and air are fluid and get rid of irregularities.

It is mostly a matter of time that the coast has been the coast and eroded only by interaction with nearby ocean which is the cause of irregular coastlines. the north was glaciated, and it created a new landscape (troughs, ridges, large sublinear scratch marks, and so one). It isn't just the coastlines that have obviously different features (lakes abound in recently glaciated terrains, for example). Only a few short thousands of years since the ice went away, so the rounding that happens when ocean meets land (eroding promontories like capes and points and filling in indents like coves and bays with sediment), and the work of long-shore drift, haven't had time to do the work well.

Basically, energy minimization is at work and energy is minimized by elimination of points and dents (just like a rock rolling in a stream gets rounded). The land/sea contact zone is always in the process of linearization and smoothing/rounding, equalization of forces of the ocean against the land. The presence of points acts to turn the waves toward the points and concentrate the energy of the ocean on those outstanding features, eroding them faster. The stuff broken off migrates to open spaces and fills them in.

Really, the same sort of process is happening everywhere that erosion by wind and water is occurring, leading to the smoothing of contact zones (elimination of zones of unequal exposure). Juvenile terrains and renewed terrains are marked by numerous irregular features. Those features do disappear with time. Basins fill in and high points wear away.