Ticketmaster is owned by Live Nation (who also owns 4-5 other ticket platforms), and LN owns many of the venues (House of Blues, for example), or has exclusive ticketing deals with them. For example, MLB.
Edit: Adding this link to the SEC that lists LiveNation companies: https://www.sec.gov/Archives/edgar/data/1335258/000119312512075895/d277780dex211.htm
Edit 2: From LN site: https://www.livenationentertainment.com/about/
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