He hasn't done it yet.
Maybe this is the year
I don't think it's likely Lampkin is going to make some big leap to being Al Horford, but sometimes players do get better. Sometimes they add skills nobody saw in them previously.
If Lampkin can add the ability to hit a three on the pick and pop, he goes from a journeyman Euro pro to an NBA rotation player. It also would be huge for Syracuse long term if a good player like Lampkin with few NBA aspirations suddenly becomes a draft pick during his time here.
It doesn't hurt Red or the team to give Lampkin a chance to show he can do more. At least not at first. If he hits a high rate in medium attempts vs. our cupcake games to start the season, say 9/25 in our first 6 games, Syracuse's ceiling gets a lot higher than if he attempts 0 and never reveals the ability.
If he misses 24 threes in the first bunch of games and we win by 15 instead of 20 the overall cost is low (crappy NET rankings notwithstanding) and Red can pull Lampkin back in and say "we tried" both to Big Eddie and also to future to transfer recruits who want to expand their game when transferring.