There are many good mobile phones are available in the market who have only internal memory.
Why biggest manufacturers such as apple, Xiaomi and even Google nexus are not giving memory expansion slots to their mobile phones?
Are they facing any technical problem in terms of OS support or Performance degradation by giving memory expansion slot?
Xiaomi put this down to design restrictions in the case of the new Mi5 - they say that there was so much to cram under the hood that they literally couldn't fit the slot in.
I believe them - I've just got the Mi5 and it's featherweight and slim.
Multiple reasons. Technical considerations like read/write speed significantly faster on eMMC than the best class 10. OEMs can guarantee the same experience and better support for everyone. Less hassle hardware and software side. It's generally a design choice, they restrict the user to save themselves headaches in tech support, ota builds, hardware design, roll-out time..etc. From OEM point of view the benefits far outweigh the costs of adding an sdcard slot.
The answer is simple, they want you to pay more for phones with more internal storage, versus buying one phone and expanding it when you run out of memory.
Perhaps because those phones already have enough (for some users) internal storage but best choice is expandable because in case your phone is wrecked or bricked you can save some of your files if the 'Expandable storage' is not damaged.