Justme's answer is sufficient: There's pins that logically can't be split passively. And no, they are not optional; especially working E-DDC is a mandatory requirement for HDMI.
Other than that: The signal lines aren't just "cables", they carry a very high-speed signal over impedance-controlled lines (typically, twisted pairs).
If you built a splitter, ...
Yes, it is possible to make a wideband power splitter with matched input and output impedances, but the signal level is reduced by half, which greatly limits the length of cable that you'll be able to drive. Also, the physical layout is very critical for flat frequency response.
There's more information here, but the basic idea is this:
simulate this ...
How can we assume the characteristic impedance of a pad is 50 ohms?
There's a similar issue whenever you connect a component to an impedance controlled trace. Whether it's an antenna, a filter, a simple DC-blocking capacitor, or the actual IC pad the trace connects to.
When the signal transitions from the controlled impedance copper trace to ...