why aren't there more musical scenes like this in anime
Because not everyone has musical talent.
And if you looked at how much the salary of an animator has dropped over the years I'm pretty sure the industry loves its money too much to hire musicians on to do much more than the standard OST.
Also since most anime is secondary to another medium, those mediums don't typically have equivalents.
So you'd need to find an anime where it's an anime original, the writer/director has the great idea to put in musical segments, and the production cost of the series is already so low that the producers can stop jewing just long enough to actually put forth the money to make it happen.
It's simple and short, meaning the animation team need some imagination to make it fill up their 20 minutes of content per episode.
It's actually the perfect source-type to receive this kind of change without mangafags getting butthurt.
I dunno how much you need to spend on anime eyes OR how much it costs to animate girls sitting around and talking, but I expect it to be on the low end of the spectrum.