Invalid nets in Eagle PCB board project
Eagle does not care what you have connected visually in the schematic. All what is of interest to Eagle is how the nets are named.

And that's what power pins as Vcc and GND are special in: They automatically name the network they are connected to as Vcc etc. So all other power pins of the same name are connected automatically. That's what you have here.

But that behaviour isn't limited to power pins. You can rename any network and segments not visually connected are still taken as connected. This is a pretty easy way to avoid the wiring mess you see in your schematic. You can use the "bus" tool to draw a single thick blue line between different parts of the schematic. Or simple place a label ("label" tool) at the net segments to show people these wires have the same net name and thus, should be considered connected. Or use the "connectors" from the dports library. The latter is useful for spreading a schematic over several pages.



I recommend you to redo your schematic. Remove all the wiring, then draw short net stubs from your I²C modules. Use the symbols from the supply library to connect your Vcc power pins to each other, same for GND. Then rename the individual SDA and SCL networks into a literal SDA and a literal SCL. They are connected automagically internally (not visually). Use the bus tool to draw a visual connection. Eagle simply ignores it, but people will understand.