I'm also in Oregon, specifically Portland. DMARCs for fiber are largely shared by providers which is why you can have multiple providers in a given area. That way they don't take up extra pole space and the city can pay a single provider to maintain sections of line.

As long as the DMARC isn't saturated then you're only limited by the other lines also going to the DMARC and the collective activity of subscribers upstream of you between you and the central office. That's to say, you are right that it can be done but only insofar as other posters have indicated: last mile providers need to provision higher capacity lines between the DMARC and the central office.

This doesn't prove anything, it just means that when you're doing a speedtest there aren't other users that are also saturating the connection.

Note that this doesn't means that there are exactly 0 users downloading in those moments. Usually there are multiple gigabits dedicated to a group of users, so that multiple users can navigate at 1Gbps without slowing down others, but not all the group at once. How much bandwidth is allocated to how many users can vary though, and some providers might allocate less total bandwidth to more users.

In practice this works out fine most of the time and most users won't notice slowdowns like you do, but if everyone started a speedtest at the same time you will notice it.

i'm in oregon and those assholes won't provide fiber to the home to my neighborhood on the far western edge of the metro area out near forest grove.