I know there's a few threads on this topic, but I'd like to add something relative.
One suggestion I'd like to offer is that these "28 Planets with Mortal Life" happen to be worlds where they are shown to be intelligent creatures. The "Mortals" are only considered eligible enough to be "Mortal Life" if they're intelligent enough.
For instance, Planet Earth, Planet Namek, Planet Zoon, Planet VegetaSadala (before they got destroyed), Planet Jung, Planet Yardrat, etc.
All of these planets and their people, regardless of how advanced or behind, have an intelligent species of people who have their own languages, culture, authority figures, forms of currency, etc. These are all high-level displays of intelligence. Jaco looked down on Earthlings and The Galactic Patrol Data report suggested that Earth was immature and far behind too, but it doesn't take away from the fact that The Earthlings are still smart enough to be considered apart of "Mortal Life."
A species that we didn't get a chance to explore more were those barbarian dinosaur creatures that Zamasu didn't take a liking to; after a lot of time passed when they jumped into the future, they seemed to have some form of culture, coordination, respect, religion (correct me if I'm wrong), and Zamasu overlooked that because something else was on his mind or Gowasu had his attention (I can't recall at the moment).
That said, we also have planets that show creatures who are not all that intelligent (Vampa). One debatable race for instance, are those creatures that were shown in Dragon Ball Minus (the ones Bardock was conquering with Leek); they look like less than intelligent creatures, right? One can say that at first glance, but they had their own advanced weaponry and what not, so perhaps they were considered intelligent and apart of "Mortal Life".
Planet Vampa is a perfect example of one of those planets not being one of the "28 Planets" Kai talked about, lmao. Planet Vampa has only two different species that feed on each other. Sure, they are smart enough in their own ways to be able feed on each other, survive, and the spiders show that they can coordinate attacks and the beasts show that they can establish friendships (Broly with Ba).
But for one, Planet Vampa isn't considered resourceful; it's a barren planet that doesn't even have any water. The only source of food come from the animalistic organisms that reside there. The species are just barbaric and live solely on instinct - perhaps Ba was an anomaly. The point being made is that overall, Planet Vampa wouldn't be said to be one of those "28 Planets with Mortal Life" because their race is not advanced intellectually. They have no language. They have no form of society. Sure, they are alive, can feel, and do things like all living organisms do, but when you compare their way of life compared to beings like Earthlings, Yardratians, etc - places who have culture, history, governments, authority, etc...they don't compare.
Zoon is a planet full of 'intelligent' life. Vampa is not.
Planet Earth is a planet full of intelligent life. Planet Popol (The Planet Kibito mentioned) {Assuming its main species/inhabitants are giant frogs}, would not be an intelligent planet.
Yakon's home planet may even be considered intelligent IF we believe that Yakon isn't a special case of an anomaly and that all the other creatures of his planet aren't mindless beasts that just feed on light when they can. If Yakon's people can talk, communicate, and have some form of authority, then they'd be considered intelligent and able to pass as "Mortal Life".
Freeza played a big part as to why this BS happened too, of course. And if one wonders how Freeza even stays in business, I believe it's because he doesn't just go after resourceful planets with signs of high intelligence that'd pass as "Mortal Life."
There could be planets with great resources (water, food, minerals, gold, etc) that have creatures that exist there that don't have a form of government. They don't have culture. They aren't intelligent. They can't speak and communicate beyond grunts, etc. These are planets that aren't in the same category that Shin was speaking on. These aren't the kind of planets you'd exactly look at and be like, "Yeah, this is an advanced race of people."
The universe is a vast place and the duty of the Kais is to create life. Not all of them are going to be like Cerealians, Earthlings, and natives of Yardrat.
These 28 planets could be spread vastly across the universe, and it's very likely that there are more that we don't know about...otherwise, what exactly is The Galactic Patrol defending the universe from asides petty criminals?