If it is not a crappy situation, the land value will increase and possibly offset the additional costs.
If you buy that from the municipality/city, it is not uncommon for there to be a clause stating that if you do not build on it, you can only return it to the municipality at most. There is nothing about profit from land value increases.
You can't plan every possible life situation in advance and avoid all risks.
You are right, but if an initial assessment of the situation involves spending one day researching and possibly an hour talking with your partner to see whether budget and wishes roughly align, then that is probably time well invested.
If in this example here one finds that a house according to the wishes of the TE with land and so on costs a total of 600 thousand, but their budget is 450 thousand, why should they buy a plot of land only to find out in 4 years that if things go badly, it will be 100,000 more, but certainly not 150,000 less.