I think you could reduce the profession of journalism to “forced transparency”, which can be a form of accountability unto itself.

In short: because it’s simply who they are by definition

Because it’s a moral obligation. The fourth estate is critical to the sustainable operation of our system of government and society. The electorate need to make informed decisions at the polls. In the same way lobbyists inform representatives the media inform the electorate.

Interpreting as "why would the media stop holding people in power accountable"...

Because it's cheaper and lets it survive.

The unspoken undercurrent is that Google and Facebook ate the ad spending that used to fund journalism, without replacing its function.

Google and Facebook don't fund journalism at scale -- that's why their profit margins stay at 23% & 40%.

Without revenue, reporters don't get paid, high quality reporting doesn't get done, and newsrooms grovel for any source of margin.

Which currently seems to be {minimal cost "journalism"} + {catering to power in exchange for access}.