It infuriates me that we basically had this as a feature, lost it, and have to sort of reimplement it with an almost illicit feel.
I can recall in the mod-late 1990s when Mainstream America was starting to go online for the first time, everyone was training their kids "never give out your real name." From that simple bit of stranger-danger paranoia, we built a lot of communities as psuedonymous by default-- your AIM "screen mame" was rarely your given name, you could have different usernames on each forum, your email address probably referenced your favourite sports team or anime character.
This inherently constrains aggressive "passive" personalization. Without an obvious canonical identity, you don't want to try to cross-profile too aggressively, because user "hakfoo" on site A may well be different from user "hakfoo" at site B, and you had to assume that any ID you tracked was limited or transient: when you go off to college or apply for a job, you're probably not going to want to be slapping "PonyGirl1987" on your resume.
I wonder if it's that the algorithms are limited to being one-track or if they're overoptimized to being one-track though. I assume they have a profile somewhere that looks like "12% coin collecting, 31% travel to Paraguay, 9% 2004 Ford Focus Repair, ..." and then they offset that data with what content produces the best revenue/engagement/metric of the week. In the process, many secondary interests simply get demoted to the pont you see nothing but tire change tutorials.