why are my phosphates so high?

I would think either your rock or sand bed. I am betting on your rock.

So as we both know, you and I have talked on many occasions about this. For anyone else that may also be having the same or similar issues, here are some things can try-

1. If your rock is old, from a previously setup tank I assume. The chances for it to leach phosphate are high. That said, they don't leach phosphate forever. Especially if your keeping up on weekly maintenance. If you've tested and narrowed down the source of phosphates to your rock, the only realistic things you can do is constantly use a phosphate removing media like granulated feric oxide or granulated aluminum oxide. I prefer the aluminum oxide because it's cleaner and removes phosphates quicker but it has it's issues. The other thing is many water changes.

2. Your RO unit, are you still testing your Tds with a hand held meter? Knowing how much Tds is getting through your membrane will help. Any time your RO unit is shut off, Tds builds up. It's called Tds kreep. As soon as water demand is there to turn your RO unit on, that high Tds water gets pushed through if you don't manually flush the membrane before each use. This not only exhausts DI resin quickly, it also shortens the life of the RO membrane itself. If your using an rodi then there should be zero Tds coming from your rodi water. If you end up finding that your getting Tds higher than zero comming from your rodi it might be time to check or replace your filters. Low water pressure can also cause higher Tds.

3. Water storage. Not many people would think their water storage containers could be a source of phosphate but it's true. There's been many dozens of discussions on other forums about plastic containers such as brute trash cans actually leaching phosphates into the pure or mixed water being stored inside. While this definitely can happen, again it's a short lived source of phosphates at best. Also it's one of the easiest sources to correct by just adding a phosphate sponge like a poly filter or media to the water container.

4. Salt mix. Most salt mixes contain trace amounts of phosphates because phosphates are actually an essential part of keeping many corals healthy. Yea seems to counter what we all think about phosphates but the symbiotic algae living inside any photosynthetic corals actually need a certain amount of phosphate to thrive. A way to figure out if it's your water source or your salt mix that is adding measurable phosphate is to test your RO water before and after adding your salt mix.

New rock can leach large amounts of phosphates also. Marco rock, some of the live cultured rock etc. Also even though you vacuum your substrate once a week you can still have sand leaching phosphate especially if it is wild or beach sand. We were just at Jays in Nh and had the phosphates in two of his live rock tanks checked and they were both running high phosphates.   We just went and talked to a guy who uses no additives to his tanks, first thing he says to one person in our group is stop the GFO it is not helping.  He also explained the reason dosing vodka or vinegar works until the bacteria grows to a proportion that the tank cannot expel it.  If your rock or sand is leaching phosphates you are shoveling shit against the tide. Get ready to buy all sorts of reactors, GFO, Carbon, Bio pellets and they usually do not correct the problem they manage it. Dong also does not try to achieve 0 phosphates as corals need some.  Depending on what size your tank is and the amount of rock you have it may be cheaper in the long run to buy new rock.

The sad part is with phosphates that high it's going to bind to your rock and substrate so when they get low it'll slowly leach back out. so it will probably be a slow process to get rid of it completely.
If your tank looks awesome then I wouldn't overly worry about it. Just put a plan into action and execute it slowly. Slow and steady wins the race.

It's gotta be leaching live rock, it makes sense considering I've added several new pieces of rock over the last several months.

I have a built in TDS meter on my BRS RO/DI and I also on occasion use a hand held meter to double-check accuracy. Getting 0 TDS from both so it's not my R/O filters.

All nitrate/phosphate readings checked out on my R/O water (well .16ppm phosphates but that doesn't explain why my tank is x15 higher) and Salt Mix water as well (which rules out the container and salt itself), so unless I'm overfeeding, which I really don't think is the case, it's gotta be either rock leaching, or possibly built up crud in hard to access areas of the tank that don't get sufficient flow... going to do some bigger water changes and start turkey-bastering the dead-flow areas to see how readings improve... might also starve my tank to rule that out.