This is a great question, horoscopes are such a staple of newspaper content that we never even think about where they came from, everybody just has one.

The beginnings of the modern newspaper 'horoscope' was the Sunday Express' horoscope for the birth of Princess Margaret in August of 1930. John Gordon hired R.H. Naylor to create a natal chart based on the position of the planets, sun and other astronomical bodies at the time of her birth. This was the standard way that a horoscope would work at the time, so not much like the modern horoscope at all. The column did prove to be popular and Gordon asked Naylor to continue writing predictions, these were mostly general predictions not necessarily about individuals.

But the public's attention was grabbed and the column morphed into one about birthday horoscopes, what a person with a birthday in the preceding week could expect for the coming year. This was a hit and in 1937 Naylor came up with the idea of using Sun Signs so that everyone could be included in the column rather than just those with a birthday that week.

The Sun Signs are the modern horoscope signs that we recognize with each one spanning roughly 30 days and being consistent every year. This idea caught on outside of the Express and in the 1930s you began to see horoscope columns appear in other newspapers both in the United Kingdom and in the United States.

So that's how we got to their popularity. Now to your second question, no most newspapers wouldn't hire their own astrologer, they would purchase their columns from a syndication company the same way that they would purchase comics, advice columns or crosswords. This model is still how most horoscopes writers make their money, and in some cases newspapers have been running the 'same' horoscope for decades, just with a different astrologer due to the death of the original writer.

Sydney Omarr is a good example of this phenomenon, he started writing "Sun Sign Horoscope" in the 1950s for the Los Angeles Times and proved popular so they sold it to other newspapers. He kept writing it until his death in 2003 (by which time the LA Times syndicate had changed hands a few times over). At that point his ex-wife Jeraldine Saunders (who created the Love Boat concept) took over the renamed column (Sydney Omarr's Astrological Forecast) until her death in 2019 at which point Helena Magi took over the column and renamed it Your Daily Astrology.