why laugh ? we all know about somethings and we all do not know other things
if you look closely at your screen human eye won't be able to see pixels above 320-330 ppi ( that's why apple calls its display as retina display )
a 720 p inch screen at 4.5 inch resolution will give you 326 ppi
if you increase the size of the screen keeping the resolution same as 720p
a 5 inch display with a 720p display will give you 290 ppi
a 5.5 inch display with a 720p resolution will give you 267ppi
so you see it will not be as sharp as 4.5 inch as it has fallen below that retina level..
so as you keep on increasing the screen size without changing resolution the screen will become less and less sharp and more grainy
so 1080p is useful only in devices having more than 5 inches display. and today's smartphones are coming in 6 inches . so in order to look as sharp as a 4.5 inch screen it needs 1080p resolution
Having said that
now there is con of 1080p also .. it requires more GPU Power and cpu to run 1080p resolution than 720p resolution
so if you have same same gpu
then a game will run slower on a 1080p resolution but will look much better and sharp
whereas if you run the same game in 720p resolution it will run faster than 1080p but will not be as sharp and as good looking as 1080p
Now in practice why manufacturers do that ? we all can survive with 300 ppi screens right .. they do it because it is a spec race competition.. the other competitor will provide the best hardware that is possible on this planet regardless of its utility or practical usage
we don't need 8 cores in a mobile or 4 gb ram but still manufacturers will give us that because their competitor is offering the same and people buy android mobiles looking at the specs sheet.. the higher the specs a phone has the better it is marketed and it will sell better .. next year galaxy s5 will have 4gb ram , 2k display , 8 core 64 bit processor ...