"Why aren't much safer Thorium Reactors being built already?"
Ever hear of the term " Technology Readiness Level "?
MSR's are (potentially) better but they have 2 issues, one technological and one economic.
PWR's are TRL9 because the USN picked up the development tab 7 decades ago. Then the USG pinned all its remaining money on the Sodium cooled fast breeder reactor (which demonstrates what happens when Physicists with no engineering understanding design stuff :-( ).
Leaving MSR's with a TRL of about 3, if they were funded.
Issue 2. Economically this is the "Gillette" business model. Reactor builders basically make their money selling the operators their fuel rods, which are a little bit incompatible with every other mfgs designs. So a design that can (loosely) be refueled by a person tipping bags of the relevant salts through a door is not going to be popular with any of the current reactor companies.
However if you can form a company to design such a reactor (or help fund one if any exist), then get a test reactor built (because getting someone to buy one "off the plan" is going to be pretty difficult) then you're home and dry.