People have been talking about this idea for maybe 20 years (that I can recall). I'm also pretty sure there've been numerous attempts at implementations because really it's not that complicated a thing to make.
None of them caught on though because when it comes down to it, most sites that successfully make money can probably make more money from advertising than they can from this microtransaction system so there's no incentive for them to adopt it.
None of them caught on because the regulatory environment makes it effectively impossible to implement efficiently and all of the attempts had to make fatal compromises in order to comply which made them useless.
None of them caught on though because when it comes down to it, most sites that successfully make money can probably make more money from advertising than they can from this microtransaction system so there's no incentive for them to adopt it.