Thanks for the replies everyone, always interesting to see\hear others thoughts... I guess I'll just let this die out though, as the answer is apparently... "because the
Mac OS is incapable of doing it."
It's not that it can't be done... Windows is absolutely able to do this, there's no question what so ever about that, and no way to argue that it can't, because Windows can do this. I believe I've seen threads where Linux users have stated that they've been able to do this as well... which means it boils down to being a limitation of the Mac OS... so I'll just end it at that and stop beleaguering the point, as well as stop bothering everyone with the question.
Regarding the question of:
"Why would your other devices need to share a Wi-Fi hotspot when they can, themselves, connect to the same Wi-Fi you're using?"
I suppose there could be many reasons; some legitimate, and some....questionable.
One reason is to use the machine's WiFi connection, ( since the Macbook doesn't have an RJ45 port built in ... and why go out and buy a dongle when you have WiFi already built in? ), to create a simple Firewall\Access Point that you can 'hide' your other devices behind when joined to a public network... basically turning it into a simple NAT.
There are also post on forums where folks are looking to share a hotel's network connection, ( apparently some still charge per device connected... wow, seriously? ), hence, they wish to join one machine via WiFi, and then redistribute the pc hosted network to their family members during their stay. Another reason I've seen a lot people wanting this sort of thing for is to work around their mobile phone carrier's imposed tethering fees. There are a lot of people out there who feel they should either be charged for a tethering plan with X amount of data, which would be the person's choice to do... or, buy a more expensive plan with unlimited data... but, they feel if they do the latter, they should not also be required to pay an additional tethering fee... so those people are doing it for that reason.
Whether or not those last two reasons are legitimate, ( I can understand both sides point of view ), they are topics of much debate on many forums... but that's for those people to sort out, although the debates \ arguments can be quite interesting to read.
Anyway... that said, I'll shut up now and stop with the question.
Thanks again all for the responses, pointers, and tips.... as well as for putting up with me too.