What is the moat of the major ticketing companies? Is it deals with venues? It is hard to rationalize how one company can even get a stranglehold on an entire market like this.

I feel like I could ping any random HN user and build something better in a week, which means it has been done many times already... why don't alternatives gain traction?

Yes exclusivity with venues. Live Nation owns and operates or invests in many many many large venues