[sagher]

Because that's how their business model works. Trying to cater most value for the lowest price range.  If they add one of their branded handsfree to the package then the price increases. Then it won't be in the competitive range. So those who want a good phone for a good price can get the phone and if need a high-quality handsfree you can buy separately. Personally, I prefer that. If I'm buying a $200 phone then won't be adding $50 or higher hands-free for that. I would buy it separately something which works for my taste as well as the price limit.

I think even branded phones trying do the same. Latest Samsung and Apple phones don't come with chargers anymore. Xiaomi has started doing the same but if you want a charger you can request it for free. Which is a good thing.  I don't mind hands-free but I really do mind the charger. You can't get a quality 20W 55W charger from a third party for cheap.

I am not agree with this. even if you even seen Infinix. the price range is almost 35 to 50 % decrease with same specs phones. with complete accessories set and work perfect. many many happy customers of Humingbird OS. 
and if you talk about future i here the news that companies now launch a branded bluetooth handsfree in box pack with phone. and the second news is to make a dlna or wifi direct connection to wireless handsfree which instantly charging via signal Amp current. but its still rumors. however if we see the acceleration of technology changes day by day. nothing should be impossible. 
No one need wired handsfree now a days. 5G is the pioneer for all these rumors. so wait for 5G and soon we see 5G tech equipped accessories.