first: neither brittanica nor investopedia are run by or written by economists. those images were made, probably in excel first, they're terrible, and then considered done. how much longer does it take to get a set of curves that line up precisely in the center versus a set of straight lines that line up precisely in the center? especially when the point you're trying to illustrate with your diagram is not (a) the more realistic shape of the utility curves that define the supply and demand curves, but is in fact (b) the equilibrium point at the intersection. it takes a lot longer to get the curves right, right? and that's not the important part of the model! better to just save 15 seconds and use straight lines, you can illustrate the curves on another diagram

that is how economists think

that is why those stupid images are there.