They do not know when it will hit but they say its expected
... and nuclear war was predicted in the 60s, but hasn't happened yet.
It is like the whole debate about reasons for climate change, or causes of Alzheimer's, etc ... there is insufficient reliable data to do more than say the situation exists and let's try and alleviate things until more data is available.
The problem is that 'experts' try to make predictions, for whatever reason, based on what they have. They need to, to move the situation along, but it doesn't mean they are correct, or that they understand the timescales concerned.
The most famous cases of working on insufficient data that I know of are:
1) figuring out why cholera was rife in London a couple of centuries ago. There were all sorts of theories - bad air, among them. It wasn't until someone mapped the incidences of cases that they realised it was a particular water source that was causing it. When that was sorted, the cases ceased.
2) in the last war the government worried about whether the V1 flying bomb had any intelligence built in, and thus whether Germany could target specific military sites. When the locations of all the V1 hits were mapped, using something with very fine granularity - ie with London split into lots of small squares - the V1 was found to be almost the perfect demonstration of random number generation. That is, they simply fell totally at random. The data is now used for statistical courses to demonstrate that.
Bottom line - the jury is out on all these tings. Do what we can day by day to care for our loved ones, try and plan as much as we can, do what we think is the right thing to do.
Wait until there is adequate data then believe the proposition being put forward.