Why can't cling film touch food while being cooked?

In a microwave, the food being heated can reach a boiling temperature (212 degrees), Cling film has a much lower melting temperature than that.
Best bet is to put a paper towel over the container.

Some plastics release toxins when heat,but really,why not use a glass or china bowl? I heat my soup in the bowl I plan to eat it from,my favourite Royal Doulton soup bowl.

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Cling Wrap is nothing but plastic and studies have shown that if cling wrap comes in contact with warm or hot food it makes the food toxic. If you want to reheat your soup without spillage and without making your microwave dirty you better cover your soup bowl with parchment paper and then heat, which is a safer option.

Firstly the film will melt, that happening, a lot of cases plastic has toxins.

Really, who wants to taste plastic soup?

I have one of those bakeware glass containers for that. Use it to even reheat chili and stews.

cling film melts before the soup gets to 99 degrees which is not even very warm.

The benzene molecule, used in many plastics, is a carcinogen (causes cancer).
It is okay with cool foods, but not with warm or hot foods.

Use a sheet of waxed paper instead, or a piece of paper towel. Or buy hard plastic microwave covers that fit over a plate or bowl and never touch the food.

Because cling film may melt and or at least will release plastic molecules into your food. Microwave safe plastic containers are cheap enough these days, who care if they get stained.

At work I heat soup almost every day. I have a Pyrex dish I use for that. Class cleans up perfect every time.

IT IS DANGEROUS because it melts into the food, and makes it inedible, or in other words, poison. What MAY work, though is to use 'baking' parchment paper, not wax paper or aluminum foil, toi line the inside of the cooking vessel, and then placing your soup in that. But, make sure your plastic container is 'MICROWAVEABLE', TOO. You don't want it to melt, either.

It melts

Heat the soup in a glass container and you will not have any problems with stains.
There is a soap that supposedly is able to remove the stains but I have not tried it.
You would not have to "wrap" the container but you would have to place the soup within the liner so really you are lining the plastic bowl rather than wrapping it.
It is not recommended that the food be in contact with the plastic wrap at high temperatures. (same could be said for some plastic bowls)

You should buy cling film that states 'microwave safe'. In any case you should avoid it touching food whilst being cooked as chemicals can migrate from the plastic to the food. I doubt it is dangerous but inadvisable.