Who says they don't? The way you are saying that makes me think you want it done from a certain perspective. Even though that itself will be a form of regulation. Regulating what journalists and talking heads can and can't speculate on.
They dont. It is not a right wing or left wing thing.. it is pretty much universal. It is often excused as "editorial prerogative", but it is often a bull**** excuse.
Take the coverage of the Trump speech in Poland.
You had the right wing media praising his presidential speech and trying to link him to Reagan yet again.
You had the left wing media basically doing nothing to challenge the right wing narrative and even some praising the speech.
What you did not have to any significant aspect, was reporting (by either side), that the crowd was bussed in and the fact that Polish government is a right wing government with deep ties to the GOP. These 2 bits of information are vital in context of the speech and the crowds. What you also did not have in the coverage of the speech, was the fact that he was basically reading up Polish history to a bunch of Poles... read the speech and you will see. Basically, key facts were left out by both sides that would have shown the true nature of the speech...and from that you can as a person make up your own mind if he is the next coming of Reagan or just a hustler.
Another example from the last few days. The US right is claiming now that Comey via a Hill article.. divulged classified material in his memos about the Trump meeting. Fine, but what very few did was question the article and especially the journalist behind the article. Not only was there the usual "unnamed sources", but the fact that the guy use to be an editor on the Washington Times, and worked for the Sinclair Broadcast Group.. should have sent up red flags instantly. Again, lack of facts and information on a story. Oh and if you did not know, the Sinclair Broadcasting Group is a pro-Trump local media organisation that pushes right wing propaganda in their news reporting.
Or the sadly classic of Brexit. The pro-Brexit media were lying through their teeth the whole campaign. The anti-Brexit were far better in reporting the facts.. problem was, that the pro-Brexit media dominates British media and the only semi independent media, aka the TV broadcasting, more than once dropped the ball. I never saw any reporting on the fact that the UK can deport unemployed EU migrants. Why? Because it is up to the British government to implement such a policy, something none of them ever have... why is that? Is it not worth the reporting time to point this out? Why was this fact left out of the debate?.. among many facts about the EU and migration.
Time and time facts are being left out so that a specific narrative can be formed by the media in question.. that is NOT THEIR JOB!. They are there to report the facts and let the people form their own opinion based on ALL THE FACTs.
You might call me naive or whatever... I believe in freedom of the press from interference from politicians and the corporate elite. We the people deserve to know the facts, not the alternative facts that the politicians want us to know and it is the job of the media to provide those facts and police the political and corporate establishment.