Yes, Ethernet supports a bus, but you need coaxial cable. The variant is called 10base2 (thin coax) and 10base5 (thick coax). It runs at 10mbps.
The problem is that if the cable is broken somewhere, the entire network stops working (even the PCs that are on the same side of the break cannot talk to each other because there is no terminator at one end of the cable.
Also, if you make a star topology using a hub (you can find hubs that work on 10mbps or 100mbps) instead of a switch you can also have collisions. The reason is that a hub forwards the packet to all ports (except the one it came from), compared to a switch which forwards the packet only to the required port. Hubs were used because switches were expensive at the time, so you only used them if you really needed the performance (and you can make a really simple 10mbps hub with a few transistors that just repeat the electrical signals). Now switches are cheap and I don't think they make hubs anymore.
A network that uses hubs (or a bus) is quite limited in speed - if you have 100M Ethernet then that is the total speed of the network, if PC1 is transferring data to PC2 and PC3 is transferring data to PC4 both transfers will be at ~50mbps (on a switch both transfers would go at 100mbps). So, if you have 100 PCs and they all use the network at the same time there will be lots of collisions and the network will be really slow. Also, it is half duplex (a PC cannot send and receive data at the same time) and relatively insecure - all PCs can see all packets and it's up to the network cards to filter the unwanted packets out (and you can set most of them to promiscuous mode and capture all packets).