Because you assume that it's easy enough for an indie developer to just make a 'Mario-like' game that could rival the stuff Nintendo did - even the stuff from 30 years ago - and I can tell you it's just not. Mario is all about good level design and level design is an artform that most designers out there just haven't mastered at all. We've had designers with tons of experience go through our tests (everyone who wants to work at Moon Studios has to do a quick test so we can check their skills) and a lot of them just aren't that great at designing spaces in the way Nintendo can. It's actually an incredibly rare talent. There's tons of designers out there who THINK they can do it, cause it looks oh so simple, but once they actually have to sit their butts down and design in that way, a lot of them suffer and stumble their way through it and the result more often than not leaves a lot to be desired.
I would bet that there's way less than 100 people in the entire western gaming industry that understand design in that way. Compare the indie output in terms of platformers from the last 10 years to games like Super Mario Bros. 3 or Super Mario World - if you understand a thing or two about design, you'll probably notice that there's a huge gap in quality between them. This is really a field where technology doesn't help you at all: Just cause we got better computers doesn't mean that anybody will do the design work for you and even though you'd think people have studied Nintendo's output and the way they do things over the years, most of them can't deliver on anywhere even near that level of quality.
If you look at the current AAA industry, I think you'll see what I mean: None of the big AAA games truly bank on good level design because (and this is me saying something controversial that really shouldn't be) most of these studios don't really have designers that are all that great and that could make successful games that are reliant upon good level design. The way most studios handle level design is more about spectacle, working a lot with environment artists to create 'interest', using open world terrain tools so that they don't have to labor over every inch, etc. - All the successful western AAA studios make games that work despite, not because of their level design. This is also why most western devs out there still love Nintendo's output, cause they're clearly better in many regards than they themselves are and there's still a lot to learn from them. Here's my super controversial bet: Even if you'd take the absolute best designers from places like, say, Naughty Dog and task them to create a game as good as Super Mario Bros. 3, I'd bet that they couldn't do it. You need a certain breed of designers that understand design on the level those Nintendo folks did and that kinda talent mostly doesn't exist in the current industry.
Thank you very much for the reply!