How to lose an aircraft?
It is certainly possible to build an aircraft using modern technology that is hard to lose. After all, aircraft have GPS receivers and passengers can watch YouTube in the cabin. All you have to do is integrate those two and send the exact GPS position to a ground station as frequently as you want.

However, the global aircraft fleet is not so capable. Some aircraft have satellite internet capability; some do not. For those that do, the internet connectivity is completely isolated from the aircraft navigation systems. (After all, you don't want any possibility that an aircraft in flight could be "hacked" by someone on the ground.)

Retrofitting an existing aircraft (which could be up to 40 years old or more) to add such capability would be a very expensive job. Retrofitting an entire fleet (with perhaps many different aircraft models) is astoundingly expensive. Existing aircraft navigation and communications systems are so good that losing an aircraft "almost never" happens (yes, it's happened at least twice this year), compared to the number of successful flights. The airlines probably consider upgrading their fleet to support this kind of continuous surveillance not worth the expense, when compared to the probability that one of their aircraft actually goes missing.