Because as bad as they are, parties remain organisations capable of drafting legislation for political objectives. And those political objectives provide a way for alternatives views to be incorporated, beyond technocracy and the lobbyists.

The way I see it, parties would morph into lobbies or think tanks. Already the parts of the organization capable of drafting documents from political parspectives are increasingly outsourced to party "foundations", or directly to think tanks. Lobbies lobby not only the legislators directly but the party apparatus and the foundations.

In most European parliamentary systems the government, through the upper tiers of the civil service, drafts legislation. It's not like in the US where legislation is introduced by parlamentarians. The function of parliament is increasingly limited to demanding accountability from the civil service. So that function would remain. And the parlamentarians would seek advice from think tanks, foundations or "parties" just like current parlamentarians now seek advice from their party apparatus or the party itself writes the legislation that the parlamentarians introduce.

With party parliamentary discipline as currently practised in many parliamentary democracies, especially when party-list proportional representation is used, individual parlamentarians have little initiative and are basically there to contribute to party votes.

Finally, parlamentarians selected by sortition would be less vulnerable to corruption. Not only they don't owe any favours to external interests for their access to the parliament, but as they are not part of a party apparatus they are less likely to exchange favours after leaving the parliament.

If you are not convinced, try it on someone who has not been entirely debauched by economics. — Piero Sraffa