Not since the 1950's they don't - you're confusing manual-focus with a manual-diaphragm, something cameras have not had in decades.
All cameras since then have had automatic diaphragms that meter and allow focus at the widest aperture, only stopping down to the 'taking-aperture' when the shutter is tripped.
The only factors governing light transmission to the focussing-screen (which is what accounts for the brightness or lack thereof) is the lens' maximum-aperture (the faster the lens, the brighter the image when focussing) and the efficiency of the mirror.
Some (most, actually) mirrors are half-silvered, allowing partial transmission to the primary exposure sensors located behind the mirror and in front of the shutter.
At the moment of capture, secondary sensors measure light reflected off the film-plane (or nowadays, the CCD) and adjust the exposure accordingly if the camera is set to any of the automatic exposure modes - in manual mode the exposure remains as set by the user.
That isn't the layout of modern cameras. It sounds like you are referring to something like the Olympus OM-2, which was a pretty much unique creation. And a long time ago, 1980.
There have been various configurations over the years, but as far as I know all contemporary DSLRs have (for many years) had the AF sensors in the base of the mirror box, fed by the piggy-back mirror on the back of the main mirror, which in turn gets its share of the light through the main mirror's semi-silvered centre section. So yes, that will steal some light from the viewfinder.
All metering is done in the pentaprism/pentamirror, including flash metering.
The OM-2 (and derivatives) was the only camera to read the ambient exposure off the film surface and/or shutter curtains, in real time. Some cameras later used a similar idea for flash metering (eg Canon T90). All this had to be abandoned with digital, due to the mirror-like surface of the sensor.
As others have pointed out, modern DSLRs are just not as good for manual focusing as old film cameras were. I have a Minolta SRT101 which is about 40 years old and, while it admitedly has a 50 f/1.7 lens which makes a big difference, the viewfinder is big, bright and it's easy to focus with the central micro-prism spot. Though I distinctly recall that old MF cameras were not so good at anything over about f/4 when the micro-prisms and split-image aids blacked out.
I just don't think it's such a priorty these days. The viewfinder is for composing the picture, and as long as the AF works okay, if money and weight can be saved with a penta-mirror instead of a prism, it will be. The viewing image with a crop format DSLR is also inherantly quite a lot smaller, which is counter-productive.