I never understood why some liken it to astrology. Could anyone explain?
I think I can. I was into astrology years before I ever heard of Myers-Briggs/Keirseyan personality typing. One main thrust of astrology--the natal horoscope--is about the very same thing as 16-types theory: defining basic types and features of human personality.
The only significant difference is the basis for interpretation. Astrology uses a map of the heavens, the theory being that the relative positions of the sun, moon, and planets at the moment of your birth can reveal a lot about your personality. And 16-types theory uses a self-assessment questionnaire which leads to a pattern of Jungian function-attitudes; and that pattern is believed to say a lot about your personality.
In both cases,
interpretation is key. Neither a natal horoscope nor a cognitive-process pattern can say anything about you directly; the chart or pattern needs to be interpreted--transformed into a description. Even then, the description will only be a general outline; as an individual, you have to "try it on for size" and see how well it fits the unique person you actually are.
Most people are pretty lazy, so they don't do much with either astrology or 16-type theory. They know their sun sign, but that's all they know about astrology. They learn their type code--and change it whenever they feel they've gotten it wrong--but then they jump right into descriptions and never bother to learn how the system works.
The fact that my sun sign is Pisces means nothing more than the fact that I prefer iNtuition over Sensing. The minimum requirement for getting anything out of your natal horoscope is sun sign + moon sign + rising sign. And the minimum requirement for getting anything out of 16-types theory is your four-letter code. Even then, there's a lot more you can get if you delve deeper and find out more.
But you still have to interpret it all. And you do that by self-searching and being aware of your experiences.
Either approach can be a good vehicle for self-discovery. Many people reject astrology because the "sky map" it's based on doesn't seem to have any scientific validity: why should the positions of planets mean anything at all? Well, it's faith-based to a large extent. But so is 16-types theory, when you stop and think about it. Nobody has scientifically verified Jung's hypothesized "function-attitudes" or "cognitive processes." Keirsey was sure those were nothing but figments of Jung's imagination.
Still, you can use most anything as a basis--the lines on your palm or whatever. The key is in the interpretation of what the signs mean to you or say about you. If you go around saying you're an INFP (or whichever type) and asking what other people's types are, you're saying "INFP" means something to you or describes something about you, and you're curious about how others understand their codes and what interpretations they've made.
Ultimately these are all just labels that we apply to ourselves to facilitate communication with each other.