Part 78: Black Robe discusses the origins of corgis and the United Kingdoms most :3: TV show.
I get the impression that in-universe, battling with a specific type theme is considered more skilled than going for an even spread of typing. This is emphasized by the fact that the Elite Four and Gym Leaders all have type themes, even if Champions often don't, so it comes off to me as both a deliberate handicap to show off how tough you are and a way to really get in sync with a single type's advantages. With that in mind, it's clear why Bede specializes, because he wants to come off as the most skilled and the most special, and why Marnie specializes, because of what her end goal is, and why Hop doesn't, because Leon doesn't and because he wants to have a good spread of aces.
Sometimes, sure, but the gym leaders in particular mainly seem to specialise in whatever type matches their hobby, especially in later games.
Also Milo's gym is probably one of if not my absolute favourite throughout the entire franchise. It's just so adorable.
Obligatory post: there's a yearly televised sheepdog competition called One Man And His Dog (title is not intentionally sexist, it's from the old folk tune 'One Man Went To Mow'). It used to be prime-time evening entertainment when I was a kid but now they only seem to show the highlights late at night on some random channel, which is a shame because it's a very cute show with a lot of very clever good boys and girls chasing their sheeps around.
Of course, corgis are more properly cattle dogs rather than sheepdogs. Which is interesting because they're a Welsh breed (divided into two sub-breeds, Pembroke and Cardigan; the Pembroke is the one people typically think of when they think corgi) and Wales has a lot more sheep than cattle, but the Welsh sheepdog is now a very rare breed.