Why a PM?

Here's a little taste of the kind of thought that sit in my head, and explains why even the briefest taste of "news" both drains me and makes me want to scream...

Are there any compelling reasons why we have a PM, and a Cabinet, which have some kind of precedence in decision making? Come to that, why do we need any aspect of an adversarial "govt vs opposition" approach? This makes a mockery of proportional representation, since all those who have voted for their local member, presumably on the basis that he or she represents local interests most effectively, are left without any kind of effective voice in decision making. It also disregards members' individual expertise.

If collegiate decisions could be made by the parliament as a whole, using the entire knowledge and skills of the group, surely the results would be more representative?

There would be no need for factional fighting, for arguments over who should lead, for secrets between or within parties which weaken the collective decision making process.