Probably because taking big engineering risk for gains is, in general, something that is only done by new companies trying to break into an existing market. Large and very old corporations don't have the motivation to do it since they are already big players in that market.
Not saying its correct that those companies aren't trying large risks, just that the perceived gain isn't as big to them as it is to SpaceX which is not a legacy space company. Tesla is similar, no way to break into the consumer car market with an ICE product because you have nothing to set yourself apart from legacy companies to gain market share, so you do something that the legacy companies aren't currently doing with electric cars.
It is a bit more than that. SpaceX has very much broken into the market now. They launch more rockets and cargo than anybody. They are also a massive company now. The difference is SpaceX is actually devoted to advancing space technology, not just using space as a way to make money from the government (taxpayer). A major goal is to get to Mars in terms of a manned mission, in addition to reducing costs. They operate on a fixed-price contract basis, which means the price is fixed and the contractor bears the risk of any cost overruns and delays. Most of Big Aerospace has used cost-plus contracts, in which any cost overruns are paid for by the government. Musk rightly recognized that such contracts destroy any incentive for efficiency by the contractors.
SpaceX also designs and builds a lot of its parts in-house because of how overpriced so many subcontractors are. In one example, a subcontractor wanted $100,000 for the part. SpaceX balked and Musk determined the part should be no more than $5000. He tasked an engineer with $5000 to design it. The engineer came back with a design slightly more than $5000 along with a detailed report explaining why. This kind of thing has apparently happened hundreds of times with SpaceX. And then of course they don't let perfect be the enemy of the good. They design workable hardware, build and actually test it, and get real world data to then fix what was wrong. If they blow up a few rockets in the process, no big deal. Their entire family of rockets has been designed, for less money than the Space Launch System (SLS) of NASA and legacy aerospace (SLS cost $25 billion, Starship so far $5 billion to develop).
The engineering and development feats of SpaceX should be credited to the engineers that proposed these solutions and worked to bring it to fruition, although im sure they will remain nameless as Musk has positioned himself as the owner of these ideas.
While Musk wanted to start a company to reduce cost of rocket launches and was willing to acquire and provide the funding, I find it hard to believe the specific solution that SpaceX has wasn't proposed to him by some engineer(s) familiar with the industry that had this idea and just needed someone to invest in it. It kind of seems like the things that are often just wild ideas that some university researchers come up with and could demonstrate in theory.
Musk was himself the one who came up with the idea for how to do a reusable rocket. Originally he sought to purchase rockets from the Russians. The Russians said no, and on the flight back, Musk says to one of his chief engineers, "Hey guys, I think we can do this ourselves." His engineer was skeptical, but upon looking at the plan, was unable to refute it. SpaceX built its own rockets from scratch and it took three launches to get a working rocket, with the first two blowing up.
SpaceX has also benefited from having access to the NASA technical archive, but legacy aerospace has had the same access.