They could or are perhaps saving the fat 600mm f/4.0 DO which likely sacrifices a wee bit of IQ for a much shorter lens.
I replied to this as a proxy for the multiple comments about money.
The biggest expense one has direct control over is the car...and that is GREATLY lessened by buying used. (On the other hand, maintenance/repair can almost make up the difference.) And of course, you bought used (unless someone is still selling Volkswagen Beetles at the early 1970s prices).
If you're able to rent a house for 3-500 quid you've got very cheap real estate in Scotland, or at least your piece of it. Rent would easily be three times as much where I live, and for a flat it could well be twice as much. And that certainly makes the difference, right there, between being able to buy a Big White once a year or so...and not.
And not having kids living with you no doubt makes a big difference too.
Personally, I could buy one, if I wanted nothing else, but I don't do that kind of photography often enough to justify it. My gear is already vastly better than I am!