It's not bizarre - I may well book a flight to Amsterdam in January, I may want to travel to Manchester one evening. Yes the former does require more planning but then you'd expect it to, you are going to a different country.
I would imagine only a very very small proportion of people often find themselves travelling on a 320 mile round trip with 10 minutes notice. Advance tickets are available right up until the night before travel - it is reasonable that the majority of people about to undertake a 320 mile round trip would probably know about it the night before. The obvious exception is people who travel for work but most of these would drive for flexibility anyway even if the train was much cheaper.
Would you really 'might want to travel to Manchester one evening'? You live in Oxford - it's 3 hours away! It's not the sort of place you might casually pop at no notice for a nice meal and if it is you are very very much in the minority.
In fact, I think a flight to Amsterdam this evening for £150 is fairly reasonable, when I still pop out my monocle to £62 Ox-Manc.
If you were to decide now to travel to Manchester on Christmas Eve - which lets face it is more likely than deciding to travel in 36 seconds time - you'd pay just £27.50 which on the face of it is quite reasonable. It would cost at least that to drive and park.
I do take your point though regarding certain trips being good value when you do have the luxury of advance notice on certain lines (I hadn't seen that post before).
As with any system you need to play it to suit you. All that would happen if walk-on fares were the same as advance fares is that we'd all pay more. You'd just get everyone on the 4pm train full and standing paying £50 rather than managing capacity by offering £20 advance tickets on the trains either side and £30 advance tickets on the 4pm train up to a certain quantity, followed by £60 walk-on tickets on the day....
I am a huge car enthusiast and love driving but I often use the train and simply wouldn't do this with a 5 Series parked on the drive if I didnt clearly feel it offers reasonable value and convenience in some circumstances. I'm not some hapless train user trying to justify choices forced on me
Neither am I saying the fare structure is perfect. There are unbeleivable quirks in it. The best one is this..
Tiverton Parkway to Taunton Single: £8.50 (15 miles)
Tiverton Parkway to Plymouth Single: £9.40 (60 miles)
Tiverton Parkway to Bristol Single: £19.50 (60 miles)
These are all walk-on fares.