Unlike Ethernet, no central hub is required. If you want to integrate wireless into an existing network, you need to invest in a wireless access point to bridge between the two. You can use ad-hoc mode in combination with an existing Ethernet, but you can't bridge them together. You would need a router with one wireless interface and one Ethernet interface, and each network needs separate blocks of IP addresses. This might make sense if your network is large enough.
Most people are probably interested in infrastructure mode.
No company can afford to simultaneously replace all their NICs with wireless ones, let alone discard all their old-fashioned CAT5 network infrastructure. Besides, a wireless network will never run as fast as a physical network. Only one packet can be transmitted over an Ethernet at a time. When two machines transmit Ethernet packets simultaneously, this is a collision. Both machines wait a random number of microseconds, and transmit the packet again.