Because rail travel takes ages and unless you see the travel day as part of your holiday or enjoy rail travel I do prefer a flight.
We often take very early morning flights, no way, even with living in SE London, could I manage to get the Eurostar to get me the same time of arrival.
I hate travelling with luggage, I do it often when I visit my mum with luggage for 4-5 days but for 2 weeks - there is just no space on trains. And I don't want to backpack/wash clothes/live on a capsule wardrobe.
I also love exploring the world, we are off to Vietnam next year, we look into Kazakhstan and New Zealand.
This year Italy - the journey would have taken 2 days at least constant travel.
I once looked up Vienna, trip via my mum, 8 hour train with 3 changes. The ticket was extraordinary as it would have meant a total flexible on as the cheaper ones are linked to a specific train and if you miss your connection - your problem.
Even in the UK - DD studies in Lincoln, she travels by train with a railcard. DH and I will visit her in December and driving is absolutely a lot cheaper.
I want a railcard just for one person but that doesn't exist for anyone under 65 for the whole of the UK.