As an automotive engineer, I think I can give you an answer.
For starters, its been around since 1973/74. With the latest iteration, it has survived 7 previous evolutions and is in its 8th life now. And with every iteration, as with every other company out there, they make things better and iron out the bugs. With the advent of electronics, it has only become more advanced, more reliable and more robust. Sure some niggles might exist here and there. But none major.
Germany and Germans, who are conservative in nature, prefer stability. For a car thats sold in the millions, standard design and been around 50 years, it doesnt get any better. Well maybe the Beetle but its weird looking and does not appeal to the family minded.
Every other OEM from the VW stable.. Mainly Audi, Skoda and Seat.. Have had their models developed having a platform in mind. Which means they share parts and design at some level. So for a car to have formed the blueprint for all of them, is an engineering marvel.
Between 2013 and 2019...
Audi A3
Audi Q2
Audi TT
SEAT León
Škoda Octavia
Škoda Superb
Volkswagen Arteon/CC
Volkswagen Atlas/Teramont
Atlas Cross Sport/Teramont X
Volkswagen Golf
Volkswagen Golf Sportsvan
Volkswagen Lamando
Volkswagen Passat/Magotan
Volkswagen Touran
From 2019 onwards...
Audi A3 Mk4
SEAT León Mk4
Škoda Octavia Mk4
Volkswagen Caddy
Volkswagen Talagon
Cupra Formentor
Ford Tourneo/Transit Connect