So according to Fourier all square waves are a sum of sine waves. This means a square wave through a low pass filter rounds the edges because the higher frequency sine waves are removed. Through a high pass filter you get spikes at the edges. This doesn't happen with pure sine waves. High and low pass filters can reduce their amplitude but the shape of the wave coming out won't be effected.

My question is why the hell did we build all digital electronics around square waves if sine is so much better? The traces in semiconductors and PCBs produce some unavoidable filtering which fuck with the shape of square waves but have no effect on sine.