Some early Saracens were brazed I believe. Think heathy on here has one actually.
Indeed the '89 Kili Flyer was one viewtopic.php?p=1539658
All the reasons for fillets are mentioned above, to summarise:
Before the days of TIG weldable steel tubesets mass produced bikes were made with lugs and brazed.
Lugs usually cast or stamped have a high startup cost investment to make them at your desired angle.
Lugged bike cheap and easy to make - don't have to mitre tubes really accurately etc.
Braze is low temperature so doesn't affect the strength of steel (before steel improved)
Custom builders can't afford to have lugs made up for low volume bikes / custom angles so use labour intensive fillet braze process.
Hence the most desirable custom frames are fillet brazed.
American mass produced MTBs start to use newer steels that can be TIG welded (high temp) as its quicker and cheaper than building with lugs.
Small custom makers stick with fillet braze as it has a reputation and looks nice, and distances them from mass production.
Most mass produced MTBs would be lugged for cost reasons, and then TIG welded when the newer steels came out. The Saracen above is the exception that proves the rule