Because the human will do a better job settling on Emp or lower and having his cities better managed. This will be less so as you go up to DG and on. Their discounts will offset your better management. Making it harder.
For instance, players will build more workers and have them doing useful things on the correct tiles. Players will (hopefully) do a better job of trading and timing of trades. They will manage and time new settlers better.
You can see the AI making a settler in the capitol at size 1 or 2 and not going to get to 3 for a time. Even, if their food is not that good, they still fire up a settler. The player will strive to get extra food to grow faster. Maybe set up a settler pump and a worker pump on a larger map. Maybe switch building settlers to a different town, if one has bonus food. Will ensure citizens work the correct tiles for whatever is needed, be it workers/settlers or units.
The larger map, tends to give the player plenty of time to gear up and not have to go to war. The human will be aware that it needs to get a buffer between towns and the fog. Maybe put a spear out to bust that fog.
Keeping barb spawns down to areas farther out.
I guess I should say a few words about settling to make it clearer. AI have cost factor of 9 on Monarch, this is not that much of a boost. So they pay 10% less than the human for things. This also means they grow a bit faster.
They trade 140, you 100. So they get a nice discount to trade ai to ai. They get many discounts and start with more units. They also have 4 extra free unit support.
These are not much, but will hurt a bit. The bump to Emp is CF 8, so 20% and growth is faster than humans and can be noticed. You have only 1 citizens born content.The extra units helps them, they get 4 def and 2 off units extra. The offense types are tied to
that civs starting techs. So Germans start with BW and WC, so they get spear's and archer's, iirc. Probably do not want to rush them right away, lol.
Type 1 and type 2 starting units do not apply to Mon, but does to Emp. They get a type 2 unit extra to start. At Emp you start with only 1 citizen born content. So the hole is a bit larger turn 1, but not large.
As to water, well the AI cannot invade over water, even on deity. On Sid they are no better, but crank out units and ships to the point that they can swamp you in the first age on islands, if they find you quickly. I had a stream of over 26 galleys coming my way on Sid, bit of a problem. On the flip side, I was in a Sid island SG with Handy iirc and we were the Byz and just kicked their butts. Dromon have lethal bombardment, just plain cheating that early in a game.