Because no test reliably disproved the null hypothesis that it is random (in a particular way, events being poisson distributed)<p><a href="https://physics.stackexchange.com/a/340552" rel="nofollow">https://physics.stackexchange.com/a/340552</a>:<p><i>Science doesn't tell us the reason things happen. It provides a way to develop models which predict what will happen. For all we know, Zeus himself personally causes every radioactive atom to decay when he sees fit. Science cannot disprove such a claim.<p>What we can do is use the scientific method. In the scientific method, we pick a "null hypothesis" which is what everybody expects to happen, and an "alternate hypothesis" which is the interesting thing we want to test. Then we run an experiment and hopefully show that the null hypothesis is highly unlikely, while our alternate hypothesis is good. In the case of this topic, the usual null hypothesis is "radioactive decay is random."<p>To date, nobody has been able to develop a test which can demonstrate that they can predict the timing of radioactive decays better than random chance. That's not to say there's not some local hidden variable, or a cherub that knocks the atom about to cause it to decay. It just says that nobody has been able to provide such a theory which does better than the "radioactive decay is truly random" theory does.</i><p>So, there may be a perfectly non random reason, but if so, that reason is similarly random. For example, if one hypothesizes a particle that ‘hits’ atoms that causes them to decay, those particles must hit atoms with a particular random distribution to explain the data we see.<p>And yes, t radio-active decay could be driven by a 100% predictable random number generator, but if so, we haven’t noticed yet, so that random number generator must be very, very good.