Why would countries have issues with Apple using their own modems?
Not issues as much as "proprietary standards". At a 100,000 foot level the 4G , 5G standards are uniform. If get down into the 'weeds' of standards implemented and deployed per country get down into varying frequencies and encoding standards.
Apple switched from Infineon to Qualcomm modems in part because Qualcomm was first to have "world ready" modems that worked "everywhere". ( some firmware options that is adaptive to all the variations. ). That problem has only grown more diverse and varied
So primary case in point China has a frequency/time encoding that countries that don't buy Chinese base station equipment can't do. They use their "we are a billion people market" to get folks to go through gyrations to deploy to their country ( plus they now have greater leverage on the products in market. Need to certify equipment in country. So have to bring it in and test. )
Furthermore, each country has their own certification/verification standard. Which means have to bring in equipment and pass muster with the government and how many telecom players are in the company. That can mean multiple bureaucracies to navigate ( and in some countries multiple layers of bribes and graft to pay). There is lots of 'scut work' to getting certifications in hundreds of systems. In part, it just takes time. Part of the substantive upside of buying Qualcomm modems is that they have already done much of the tedious work of getting certifications and approvals. They have the service and base station provider relationships. Ther is a mountain of i's to dot and t's to cross.
For Apple there are probably diminishing returns for some countries. To much "drama" and not enough sales to put the modems on the "fast track" certifications.