With a (predicted) $5.5 Billion dollar market, why isn't SpaceX trying to build its own 'mini launch vehicles'? With the brand and the technology they have, they can easily capture a huge chunk of the market.
They already have one. The Falcon 9. Earlier this month a single Falcon 9 put 64 smallsats on orbit. It was arranged by a rideshare company, Spaceflight, at prices that small launch vehicles would have a hard time competing with, starting at \$300,000.
Everything SpaceX does is in service of Elon Musk's ultimate goal of retiring to Mars. The Falcon 9 was a stepping stone, developing a separate small launcher would take a lot of development effort that doesn't help the ultimate goal.
Their plan involves moving all payloads to the BFR, a reusable spaceship that could be more economical than throwing away a small rocket on every launch.
They already dipped their toes in that, it was called Falcon 1. Basically they didn't find enough demand to justify keeping it around. Granted today there is more of a demand for such then there was in the past, but...
The ultimate solution will be Starship (BFR). The estimated cost of a single launch is about \$6 million. An Electron rocket costs \$5 million. The amount of payload that Starship can take to LEO is VASTLY larger then an Electron rocket.
As for the \$5.5 billion market size, that would include building and launching them. Let's say there is 100 launches per year, that would only come to a \$1 billion launch cost of total launches per year. They could do it, but there is a limit to how much engineering they can do, and it is more worth their while to go after the bigger fishes of Starlink and Starship.