That game's Online makes me depressed. Not only couldn't we get 4 friends into the same lobby for some reason (the last person to try to enter could never get in, no matter who that was), but there's no option to add CPU opponents to online matches with friends. And Mario Kart functions very differently without a certain number of players in the rear positions to pick up the constant red and blue shells, stars, lightning bolts, etc, to constantly switch up the positioning.
If you play two-three players alone, a very possible scenario is that the person that takes the lead never gets passed even once, because the people in position 2 and 3 won't get enough items to take out the banana/green shell shield the leader has while also attacking each other at the same time. And this actually happened every time we played, as we took turns to win without any competition or interaction with each other. It was quite boring.

I don't recall exactly what it was now, but there was some reason for why we didn't. Perhaps it was the "getting everyone into the same lobby" issue there as well, or maybe we just wanted to play with people we knew.

I was complainign about both not being able to play with all my friends, and about not having enough players on the track.
As for the latter, there are several reasons for why you some times may only want to play against people you know. If you're in a worldwide lobby, you become dependant on these other people not leaving (or disconnecting), and sticking around for as long as you guys want to play. Which creates more waiting periods in between matches if you try to find new players to replace them. Not to mention the initial waiting period when you join, which can be either short or long. Then there's the chance that the worldwide game starts before all of you get in, and you have to exit back out and try to find another worldwide lobbty. (I kinda recall that being what happened, but I'm not sure. It was in June 2013. Maybe my friends can remind me if I ask them.)

Another reason you may only want your friends to be your actual opponents is that you want to know who beat you. Someone who can trashtalk you over a conference call while you're playing, or vice versa. It's not the same as when a random actual person who you don't know wins. The CPU is not seen as an actual person that can outsmart you. They're just there as filler, and if they win, it's more comical than anything else.

I don't know what the issue was with all of us not being able to enter the same created private lobby. But I was happy to hear that you can add CPU opponents to online friends games in MK8. Just wish they let you do that in MK7.
Since I bought the game, I've played it maybe 3 times. It's the only game I ever wanted to return in 20+ years of gaming, along with Call of Duty: Black Ops. (Which I was talked into buying.)
The single player mode in MK7 was great though. I just bought it expecting to play with my friends during lunch breaks.
I ended up keeping the game though for some unknown reason. The Rainbow Ride music was too good.

I already bought MK8, just no WiiU yet...

I got it for $40, but that's actually normal on the site I buy from. That wasn't why I got it. Remember that deal where you got a free game if you bought Mario Kart 8 and registered it before September? That's why I bought it. Because at the time I thought that I would get a WiiU by September, but I waited a bit just in case there would be a nice bundle, or if I would find a better price (if not official price cut, then just some local deal). But as time passed I instead became hesitant to get the console, to my surprise. Long story short, the cost of the console vs the amount of games I'm interested in getting for it don't quite match at this point.