No particular reason. I wouldn't like to say which is more common, but there's absolutely nothing unusual about the plural version 70 percent of the population of the world own mobile phones.
– FumbleFingersJan 22 '22 at 13:31
It has a distributive meaning, where "a mobile phone" (singular) is owned by each member of the set of 70 percent of the population.
– BillJJan 22 '22 at 13:41
"70 percent of the population of the world own a mobile phone" uses the distributive singular and implies (in the less rigorous sense) a 1-to-1 relation more strongly than "70 percent of the population of the world own mobile phones" does. However, neither demand a 1-to-1 or an each-person-to-many-phones ('many' here = 2+) relation.
– Edwin AshworthJan 22 '22 at 15:02