Why can't you say "ayer he visto a Marcos"?

You can only use the perfect tense, "haber + perfect participle" when something is either very recent or is still ongoing. "Ayer", yesterday, is definitely not still going on, and it's not recent enough to justify the perfect tense. When something is final and completed, like yesterday is (!) we have to use the simple past - so, "ayer ví a Marcos".