Why interfaces and buses gets faster with every version if they are physically the same?
They are multiples ways to improve the bandwidth of an interface or bus:


Overhead encoding : most of old buses use 8B/10B encoding while newer interfaces or bus versions will use 64B/66B and PCIe 3.0 even use 128B/130B encoding.
With 8B/10B you send 20% more data than the useful ones. With 64/66B the overhead ratio is only 3.125%.
What does that mean ? In general we talk about useful bandwidth about buses. So for a 1 Gbps bus, if you use 8B/10B, the real bandwidth needed is 1.25 Gbps.
But the overhead encoding is dependant of the following evolution.
Moore's Law factor: as time flies, you'll have more computing power in a chip and you'll be able to use more powerful algorithms but which need more computing power.
You also will be able to make your chip consume less power or having more powerful transceivers.
You will also be able to increase the frequency of your bus.
Transceivers: Even if we talk about digital bus, the signal transmitted still need an analog part. This is done by transceivers. They are very power consuming parts of an IC and with the Moore's law and shrinking, you'll be able to have more powerful transceivers than few years behind.
You will also have more evolved equalization and pre-emphasis algorithms that will provide the signal to be sent on a longer distance or the same distance but with a higher frequency)
Modulation: Ethernet standards use different Pulse-amplitude modulation (PAM-5, PAM-16) to be able to send more data on the same 4 four pairs cable.
DDR: Use both clock edges to sample data.


This is just some facts that come to my mind.

Why can't we have a very high bandwidth at the beginning ? Try the Moore's law.
I think you may be able to have the best of the best in equalization, encoding, modulation, but you'll need a chip that will be very expensive and very power consuming.

Also, for instance PCI Express 1.0 started in 2003 with a bandwidth of 2 Gbit/s per lane. Ten years later, with PCI Express 4.0 we have almost 16Gbit/s per lane. But did we need 16 Gbit/s bus in 2003 ?

Consumer electronics need affordable solutions for their needs. If you try to sell them a 10 Gbps bus for $15 when they need only 5 Gbps for $10, they won't buy it.
But bus maker know that there is a higher bandwidth need as time passes and they prepare the evolution to be cost-effective.