Gromit has it spot on. It’s all rigged (commercially not corruptly) so you’ll never get four ‘big’ teams in the same group. Same as seeding in other sports. You’ll generally end up with one huge earner, a couple of mediocre teams who can throw up a surprise, and a whipping boy. All to maximise the interest. It’s the same for the Euros & the world cup.
If you had Real, Barca, Man U and one of the Milan clubs in the same group you’re guaranteed to lose interest from a vast swathe of supporters after the group stages.