I doubt Valve decided on the protagonist recently. This has to have been years in the making

I can see that happening but consider this; the closest "character" they have to "player character" with a voice is Alyx.

As sibling posts are pointing out, that's not new for Valve at all. They have been doing this for decades at this point.

Lara Croft would like a word.

Or Samus, if you want to go back even further.