Sealed beam lamps existed because of the lamp technology available. The technology came out around about the Second World War, at the time superior to attempts by vehicle manufacturers to make lights but fairly awful by today's standards. Yes there were developments but really it was an old old technology even when BL adopted it for many cars. Typical of British Leyland/Rover really to use an off the shelf unit, they didn't need anything fancy, just a holder and the optics and reflector was all taken care of.
You may have noticed that filament lamps gradually became smaller, take the original B22d/ES22 100W GLS lamp it was pretty big, then they got smaller and smaller until we now have a little capsule inside the smaller GLS shape glass bubble.
All old vehicle lamps used to be bigger too - it was to do with filament technology, evacuated bubbles or inert gas filled etc.
Sealed beam unit is the 7 inch unit, complete with reflector and the spade terminals on the rear. When it blows you toss it out.
Sealed beam conversion kits are like the Wipac Crystal unit that you have the same shape of body, with usually an H4 lamp in it, to re-lamp it you just remove the lamp from the lamp holder on the rear which resembled any other car headlight lamp renewal process - that isn't a sealed beam unit.
