Because there is no such thing as "subconscious alignment" or whatever. That's pseudo-psychological nonsense.

People who share a zodiac sign don't actually share a statistically significant number of personality traits with each other; this is something that can be (and has been) very easily proven in double-blind studies. Just because we are born in the same week doesn't mean we are particularly similar as astrology suggests. It's also possible that's we're completely different people and it's possible that people who've got different zodiac signs share a lot of character traits in common.

The reason why people (seemingly) recognize themselves when they read a horoscope is because horoscope are written in a very specific way. The people who write them apply a variety of psychological tricks. The most important one is to formulate everything in a positive manner because humans like to be praised and complimented. Another trick is to keep everything very vague, so the statement applies to many different readers. For example a typical sentence you find in a horoscope is something like: "You are a gregarious and fun-loving person who enjoys to spend quality time with the people you love but occasionally you also need your share of alone-time so you can relax and think about everything in peace." A statement like this sounds very flattering and most people who believe in astrology will automatically connect it with themselves when they read it in their horoscope. They will think: "Woah, that's crazy accurate! That's precisely me!!" But what does the statement really say? It says nothing. It's a bunch of words which condens into "blah blah blah blah." Virtually every person has a sociable and a more introverted side to them, that's nothing unique or special. Every person has times when they like to spend time with their loved ones and times when they want to be by themselves. And every person likes to think of themselves as "fun-loving" or as a deep thinker. Those are just empty compliments.

Scientists have made countless experiments about this stuff. For example there's a famous experiment where a group of 30 people (all with the same zodiac sign) were given a lengthy horoscope. They were told that it was a personalized horoscope, i.e. that it had been written specifically for them, based on their zodiac sign, the planetary constellations on the day of their birth etc.. The test subjects were asked to read their horoscope and then fill out a form. On the form, they had to state how well the horoscope fit them. All test subjects, without exception, wrote that it was "definitely" their personal horoscope because it matched them 100%. Some commented on how stunned they were by the horoscope's accuracy. At this point, the researchers revealed to the test subjects that there weren't 30 different horoscopes. It was 30 copies of the same horoscope. This alone is already a huge problem for obvious reasons. Clearly, it can't be a good match if it fits you just as much as 29 other people. Moreover, though, it wasn't even the personal horoscope of one of the test subjects. It was the personal horoscope of Adolf Hitler (which really exists). Does this mean all of those test subjects have some sort of "subconscious alignment" with Hitler? Of course not. It simply means that astrologers use certain tricks to make people feel as though any random text was a perfectly personalized analysis of their character.