In a word, SAFE.
That's why stations do liners, the stations don't want the jock to ad lib comments, and by having them read literally every word you want said on air, you eliminate any problems with the FCC due to some "on air" person deciding to try to be Howard Stern and offending some of their audience. Remember we live in an era of Political Correctness and law suits. We didn't have either in the 1950's and 60's, which might also help explain why jocks had more freedom then.
Stations have a computerized playlist so the jock can't improvise and add other music. Ah, the good ole days when a jock had that freedom, but alas the stations pay consultants today that tell them what songs appeal specifically to a specific audience. The great fear is, you play one "clinker" and you might lose most of your audience as they station surf to your competitor and may not come back. By being safe, management can control the station's "sound" with no deviations even though that "sound" is very vanilla to many of us and boring. It is a form of branding. This apparently pleases the target audience the advertisers want, WOMEN.
When you think about it, basically it's like someone taping music and inserting some comments by some jock and then playing the tape back on the air (essentially that's what voice tracking is). Most younger people, that I've talked to, don't want the ole Top 40 jock. They find that style humorous. It depends on the person, many don't want the jock to joke around or have a conversation with a co-host. Many just want the music and maybe the weather and traffic. They don't want to be entertained. They basically want a various artist CD of their favorite artists played on the radio. Yet, there are many who do like humor and thoughtful commentary or comments about some weird stuff in the news or Hollywood, etc, after a music set. So it depends on what audience your sponsors are seeking. Remember, the sponsors (advertisers) are your customer, not the listeners. That may be why some stations do allow their morning jocks to joke around or have a side kick to talk to, etc. They are going after THAT part of the audience that likes to have some entertainment along with their music. Those bland boring vanilla sounding stations are going after the part of the audience that indeed does not want a lot of chatter.
I'm not saying I like it, but that seems to be the reality of the business. Most FM stations could be automated like the "elevator music" stations were years ago. I remember seeing the WDEL-FM studio one time(today's WSTW). All it consisted of then was a giant computerized tape deck that had three large reel to reel tapes so it could mix the music between the three tapes, plus it had a carosel of carts for spots, weather, local news headlines, local ID's, etc. This device also had the ability to fad out the music and do a legal ID and switch to NBC for the hourly network newscast. Every three hours, if I remember correctly, someone would come in and change a tape and possibly pull and add carts. Basically it was the less hi-tech version of voice tracking that goes on today. But basically that's what pop FM radio has become, even when there's a live person sitting there speaking on air. The only difference is you'll not hear Percy Faith or the Living Strings.
