In his exact role, for the Warriors, he's an amazing center. Every year in the playoffs, when almost every Center gets played off the floor, for or against the Warriors, Loon sticks around. His rebounding can be dominant, his connective passing is a huge asset consistently making the right decision, and he's able to move his feet with smaller guards better than almost any center in the league. If you have Stephen Curry on your team, he might be close to top-10 on your rankings. He just thoroughly outplayed Sabonis in a playoff series, and Sabonis is likely going to be the 3rd team all-NBA center.
But 29 of 30 teams don't have Steph Curry. Without him, it's a lot harder to survive getting zero spacing from your center. Right now, teams will play off Looney when he's 15 feet or more from the hoop, and he can punish them by setting a screen for Steph as he drops the ball to him and suddenly they're wishing they had their big man out at the level of the screen to take away the three. On most teams, that's not nearly as concerning a threat, and defenses could more safely keep their center under the hoop to clog up the whole offense when Looney was out there.
Just to quantify the lack of spacing a bit: if you sort by
average shot distance, Looney is 13th among qualifying players, at 3.5 feet. He's the only one of those guys who doesn't provide "vertical spacing" and gravity in that way, either; everyone above him has dunks on at least 25% of their shots, often 30-40% of them, while Looney's at 17.7%. But the bigger issue is that even that 3.5 feet seriously overstates it. Chase Center has a massive scorekeeper bias when it comes to shot distance scoring. If you look at
percentage of FGA that are scored as having been taken from 0-3 feet, GS is last at 17.7%; second is 19.4%, and league average is 24.6%. Look at opponent shooting, and it's the same thing; GS is dead last at only 16.3% of shots scored as being within 0-3 feet, with the second-fewest being 19.9%. I don't have the home-away splits at my fingertips, but looking at individual games very much bears this out. It's not that GS just has a completely different offensive *and defensive* shot distribution from the rest of the league; rather, they're close to league-average on those numbers on the road, and way, way below them at home, to get to their league-lowest averages on both ends of the floor. Half of Looney's games are in the one arena that is by far the most likely to score even an at-the-rim layup as coming from 6 feet away, and that alone is why he isn't right at the very bottom of the list in terms of average shot distance.
In terms of how much gravity and spacing a player provides with their shooting, Looney might be dead last. He's done a great job of covering for that by punishing defenses that ignore him with his offensive rebounding, screening, and passing, but a lot of that is fairly situation-dependent. If you were drafting a new team from scratch, there's a lot of team contexts where Looney might not work as well as he does now, and you have to take that into account when considering where a guy ranks in the league as a whole.