The advanced metrics for basketball really aren't very good. Basketball is just so much harder to apply meaningful numbers to than baseball, for some obvious reasons and some less obvious ones.
There's very little subjectivity w/ baseball stats. There is a MASSIVE amount of subjectivity and intentional inaccuracy with basketball stats. If you ever want to be amused (or horrified), go back and do stats for an NBA game and compare your results to NBA results. There will be a significant discrepancy.
So you have bad/inaccurate initial info (the garbage that the NBA puts out as box scores and everyone else bases their advanced numbers on) and the mere fact that there are a lot of things that aren't easily measured (like shot creation, as Seth mentioned).
Personal story: I was the head gameday stats accumulation guy for the Grizzlies for their first three seasons. When I was preparing for the role the summer before their inaugural year, the NBA implemented a new software program for stats entry. We had league-wide training in Detroit. When there, I spent most of my non-classroom/training time in my hotel room with a sweet A/V suite doing past games myself, then comparing them line-by-line with the actual stats. IIRC, there are around 400ish entered/recorded events per NBA game on average. I think at the time I calculated that the average NBA stat crew had about 20 unintentional errors per game - missing events, wrong players getting credit unintentionally. (For those who care, I had myself and the Griz folks down to 3-4 unintentional errors per game and I usually knew what they were and was able to go back and fix them before submitting the file to the NBA.)
Anyway...on top of that ~20 errors per game, you have over double that in intentional errors. By intentional errors, I mean events that never happened (eg. loose ball rebound is deflected out of bounds by visiting team, instead of correct call - team rebound home team - you award the rebound to a home player in the viscinity...or fake blocks - among the easiest things to make up, next to steals and assists)...or events that are awarded to the wrong player (rebounds, steals, turnovers are the most common). The intentional errors are organizationally sanctioned/encouraged - they increase national media coverage/interest and increase your franchise's and player's visibility. There is also league pressure to protect/enhance the stats of the elite players. For example, I would guess that Stockton got between 1 and 2 assists per game for free. Partly because I disagreed with the blatant stat manipulation (that I did) and partly because I'm a Laker fan, I gave Nick Van Exel like 23 assists one game. If he was vaguely close to a guy making a shot, I found a way to give him an assist. Afterwards, I fully expected someone to talk to me about it. Indeed they did. A senior management guy - "great job Alex, that'll get this game on Sportscenter tomorrow morning!" We (VAN) lost badly, of course.
I also got %$@@$%# out by an Atlanta management guy because he felt I hadn't hooked Mutombo up enough w/ blocks in a particular first half. (I hadn't - I didn't like him because he was partly responsible for beating the Sonics and because I thought he was a bit of a punk so I made sure he didn't get a singly block that I wasn't sure he'd gotten - which was one in that half.) I told the management guy that the box score reflected the game and if Mutombo wanted more blocks, he needed to earn them. About 5 minutes later, Deke walked out for pregame warmups, asked the official scorer (the person who enters fouls and points in the archaic official scorebook) who does stats, she kindly pointed him to me, and he proceeded to glare at me for about a minute (which is, imo, a really long time for a gigantic man to glare at you). I want to say he blocked three 2nd-half shots and after each one, he made a point of, um, ensuring that I'd gotten them.