I actually didn’t look hard enough: there is a proposal[0] for something that could be called ‘BigDecimal’ in ECMAScript, but it’s a stage 0 proposal[1]. As for why a decimal type hasn’t progressed further in ECMAScript you’ll want to read the notes [2] from the meeting where the latest proposal was discussed.
Sure, lots of other factors are important, but ultimately factors like user base growth and active users are just showing WhatsApp gaining a larger revenue stream.
WhatsApp's revenue that they earned from that user base is what was driving the valuation from the perspective of everyone not in Facebook's camp. Facebook woukdnt have been able to buy WhatsApp if WhatsApp thought they could make more money off that username than Facebook.
Otherwise, Node.js can do exactly what you just said Python and MS Basic can do.