I think the coaching staff realized early on that Lauri has the perfect personality to be a "supporting starter". He doesn't get the jitters in big moments like some bench dude might because he's started his whole career, BUT he also doesn't need the ball that much. He tries to do the little things that keep the machine working while also being ready to step up when he has to. That's not necessarily an easy role to play.
Not everyone can be a 1st or 2nd option in a starting lineup. You need one or two guys there who are pretty much content to take something of a back seat. There's only so many basketballs and just 24 seconds on the shot clock. Replacing Markkanen with some "name" dude who would come in with an expectation of getting a certain amount of touches and shots on the wing wouldn't necessarily work if we want to keep developing the Garland-Mobley-Allen trio.
The chemistry of a lineup is a fickle thing. This is the same reason why I always frown when I see 3 stars joining forces to form a "superteam". Fans look at their combined stats and say "Jesus, they're gonna kill everyone now!" but in reality, one of those players will always have to sacrifice. The NBA game moves fast and even the 3rd offensive option in a lineup is often just a fallback option, never mind the 4th.
Coming back to Lauri – the other side of the coin is that he doesn't really have the personality (or skills, for that matter) to play Love's "instant scoring" role off the bench. Lauri needs a framework around him, he's not a guy who you just hand the ball and tell him to go to work.