It's not reasonable to ask them why they're moving, essentially
I disagree, someone's motives for moving can be very relevant to the conveyancing chance of getting to completion.
Essentially, when you buy a house, the vendor is very much part of the process (buyer too, of course) - for example, are they selling because it's a probate property?
Then you need evidence probate has been granted. Perhaps the seller may stall because the property has sentimental value.
Are they selling due to divorce? This can be a very tricky, emotional situation for the vendors and one or both can be reluctant to sell...
Are they upsizing/downsizing and committed to selling?
And obviously you want to know if they're selling because of a shitty neighbour - you don't want to inherit problems.
With one house I bought, from what the seller provided I had reason to believe they were actually causing the issues and I went ahead with the purchase. I was thankfully right and I had no issues.
You want to know, as best you can, than the seller is committed and you're not wasting your time and money, or buying a property you might regret. You'll never know with any certainty but asking questions helps.
So for me, personally, a seller refusing to answer anything is a red flag.
But that's just me - you disagreeing doesn't make you wrong, it's just my experience of conveyancing.