I'm A Banana
Potassium-Rich
Well, D&D has always had pretty rich rules for swinging swords. It doesn't just rely on everyone's shared sense of how sword fighting works. I'm not sure why burning field would be different.
I mean, have you ever burned a field?
To examine why these two things are different, one first needs a good appreciation of the reason why any rules exist. Pretty detailed rules for sword fighting in a game of fantasy adventure is something with a lot of utility to the players. Detailed procedural steps for setting fire to a field aren't as necessary for the audience and intended use of the game. One could be very abstract about combat and very concrete about setting fires. Or very concrete about social interaction. Or very concrete about managing a fantasy business. Or very concrete about wound simulation. By and large, these are less common needs for the audience and intended use of the game. Some application of in-world "logic" that's acceptable to the table is often the most playable solution and it keeps everyone content and engaged. Some application of in-world logic is sometimes enough for combat, spellcasting, monsters, etc., too, but there is more utility and demand for more detail there.
If that logic is fairy-tale logic, you as a player can reasonably say "Well, Jack is an impetuous youth and there's a castle on this cloud, so I bet I can walk on this cloud...". If the DM then dropped Jack to his untimely death and said, "It should be obvious! You can't walk on clouds!", that DM just broke with the logic of the game you were using.
If that logic is naturalistic logic, you might not have any reason to think you can walk on the cloud until something shows that you can (maybe you see a castle on it, so you drop a coin onto it to test that, and it stays!). A cloud you can walk on is an exception to what you might logically imagine to be true about clouds!
Walking across a solid cloud in the sky is a very different experience in these different modes of logic! (Largely just "Wow, so magical!" in the fairy-tale logic, but fraught with trepidation, fear, and cautious testing in the other)
Because D&D games usually use a combo, the dialogue between the players and DM and the mutual trust of both (as well as clear declaration of intent) is pretty key.