Some phones have 4 bands of GSM and 4 (or even 5) bands of 3g.
Most 4g phones have just a few bands
why dont the manufacturers make all the bands available - at least for 2g/3g).
Trying to find phones that work in US or Mex are hard to find
But I'll look cause ATT or TelCel offerings are very expensive.
Your guidance please.....
To answer your question, they do not do it because it is expensive, not all SOC's support all frequencies, and it raises the price of the device. Also, many devices are simply not made for the US/Mexico market.
Now, this being said, the bulk of these China devices are 2g quad-band phones. No problem there.
As for 3g (HSPA+), not all of these devices have what you need, but there are tons that do. For ATT in the United States, the only frequencies you really need to concern yourself about for 2g/3g is 850 and 1900.
4g/LTE on ATT (or at least ATT towers - which include offerings from Cricket, SmartTalk, and Net10) is a different beast. To get a 100% 4g phone everywhere there is service, you will need bands 2,4,5,17 (1900,1700,850,700MHZ) with 700 and 1700 being by far the most commonly used. This is very difficult to find!
TelCell, as far as I know, uses the exact same GSM frequencies as ATT. Carlos Slim is no dummy!
So, basically, it is somewhat hopeless to find an LTE/4g compatible China phone. It is, however, relatively easy to find the minimum of what you need for 2g and 3g/HSPA+
You need:
2g: 850 and 1900
3g: 850 and 1900
Everything else is not really needed for your purposes. I don't know what your price range is, but it can be as cheap or expensive as you see fit.
An example of a device with the above 2g and 3g frequencies from a decent company that is pretty darn inexpensive for what you are getting:
THL 4000, 4.7" QHD screen, 4000mah battery, 1GB/8GB, 6582 quad core, OK (but not great camera), etc. at Gearbest.com for under $85 (with coupon code 4000LM), including shipping.
+ one for the THL 4000. Best value out there with worldwide frequencies and awesome battery life.
They don't .. Cause you won't buy . it's that simple