The answer to this is pretty simple. Ticket sales on weekdays are second to none compared to anything from Friday to Sunday. It’s way more profitable to have a movie be released on Thursdays or Fridays so that they can make the most money with the initial hype. It’s just ideal, and opening weekends are the biggest way a movie’s success is gauged.
As for why there seems to be a lot on specific days and the releases aren’t more spread out, is because specific times of the year are big hotspots for ticket sales. For example, the movie IT was the highest selling movie in any September ever. This is majorly due to the fact that not many big producers would have their movie be released in September. The most popular times of the year are most of summer and anytime near Christmas.
It could also be that certain weekends are picked over others because companies have other movies coming out after that and don’t want to force movie goers to pick between their own movies. A staggered release date for one particular producer would alleviate movie cannibalism.