One very important factor that is not discussed regularly today: most new-hire college professors, who are also full-time or tenure track, have a doctoral degree in the field (or a related field) to which they are being hired to teach. While it is possible to be hired without one, it is rare, and usually only adjunct positions are available, which are part-time at hourly pay, per semester.
Why is this true? Well, it looks innocent from the surface. In order to teach an undergraduate subject (any subject), most colleges require that the professor have a minimum of 15 graduate credit hours on their transcript. As most classes are 3 credits, this means, on average, the professor took a minimum of 5 classes for the subject that s/he will teach.
The average master's degree is 32 (thesis) or 36 (research or recital) credit hours. While this may appear enough to get two subjects, it is not. A musicology master's degree might require 6 credits of research/thesis, 2 of piano, 6 of music theory, 3-6 of ethnomusicology, and 18 of musicology, creating a 36 hour degree. In this degree example, only the subject of musicology received 15 or more credits; this person would not have enough credits to be allowed teach piano, music theory, or ethnomusicology without taking more classes.
All broad subjects do this same type of breakdown: art divides into painting, sculpting, interior design (architectural drawing), drawing, portraiture, etc. English has creative writing, literature, reading, english for education (how to teach english to children), etc. Most doctoral degrees have 90 credits, so earning enough credits to be capable of completing the requirements to later be allowed to teach more than one subject is much easier for doctoral students.
Once you apply this information to the hiring process of new professors, it becomes obvious why most new hires hold doctoral degrees even if the job posting states that the PhD was not required. 15 credits in one subject is not enough for a master's degree, let alone a doctorate. But the job requirements are often combinations of subjects, like "US History, Louisiana History, and US Government" (that's 3 subjects, or 45 credit minimum); or perhaps the job requires knowledge in Choir, Conducting, Vocal performance, and Music Theory (that is at least 3 subjects, and becomes 4 if choir and conducting are considered two separate things, or 45-60 credit minimum).
Therefore, if you want to teach college as your main source of income, either acquire a doctoral degree or plan on becoming part of administration (they are allowed to earn their PhDs as they go, and their college workplace helps to pay for it).