Regarding dual SIM - I think the question is why most people in India have separate work and personal phones. There aren't that many jobs where you would need a "work" phone, and even for those, your employer would either subsidize your phone bill, or just require you to have a phone as part of being employed. I assume part of the difference is that here most plans have unlimited nationwide calls, so your employer wouldn't care if you make personal calls on your work-subsidized phone (and likewise, you wouldn't care financially about taking work calls on your phone). An exception would be if you don't want to use your number for work calls (e.g. for privacy, if you deal with clients directly). But even that is uncommon (a lot of people just use their personal phone anyway), and even if it's the case, there are call forwarding/caller ID solutions that will solve that.

The only cases I can think that would require a separate work phone would be when device security is supertight (e.g. you can't install apps or browse the web on your work phone) - but in that case, you'd need separate work/personal devices - a dual SIM wouldn't solve it. There just aren't that many dual-line use cases here.