Also, it's something where you can use a word like "some" without turning it into a plural: you can have "some milk" where you would have "some bricks" (rather than "some brick"). It's complicated a little by the fact that English commonly omits portion descriptions (you could say "some milks" and mean "some glasses of milk"), but in those cases the portion is implied but not actually verbalised.
But the general idea is that it's something you think about as a continuous entity rather than discrete units.
I would not call it wrong to use reklamen, although it does sound a liiitle bit more off in my opinion. If we use reklamen, it feels like we need to add something more since we are being more specific.
- Jag är trött på all den här reklamen - I am tired of all these commercials
In this sentence, we use reklam in a very general sense, we are not tired of any specific commercials, we are just tired of commercials in general.
There might be some kind of grammatical rule that explains this a lot better that I can so if anyone knows a more general rule, please comment!