Can I2C protocol employ differential signalling for a longer distance?
It was easy enough to find out there are indeed differential I2C transceivers, for example https://www.nxp.com/docs/en/data-sheet/PCA9615.pdf

The reason regular I2C is not differential is that it's primarily intended for short distances within the same PCB. To quote the datasheet of above mentioned PCA9615: "The SMBus/I2C-bus was conceived as a simple slow speed digital link for short runs,
typically on a single PCB or between adjacent PCBs with a common ground connection."

Every extra feature comes with a penalty in hardware costs and complexity. Without knowing the exact application of long-distance communication, any particular solution might be too expensive, too power-hungry, not support multi-drop connections, etc. For the same reason, there's not only RS485 but also RS232, UART, Ethernet, etc. Different solutions optimize for different usage scenarios.