California has extended holds beyond that, but using them on high profile people isn’t easy with the amount of public scrutiny.
They all have related codes. 5150 is the initial 72 hour hold for assessment. After that there’s 5250 (first 14 day intensive treatment hold), 5260 (second 14 day hold), and 5270 (30 day hold for severely ill patients). There’s also a 5300 (long term care) that lasts for up to 180 days. That’s the one often used when someone has a severe mental illness that they are struggling to get under control with treatment, and as I recall they assess after the 180 days and can issue another 5300 if needed.
So there are lots of options beyond that 14 days, but not ones they’d want to use on celebrities for fear of backlash. Especially since she’s just in her own home, not causing problems in public.