I was thinking in start playing WoW, but after talking to some friends they made me give up on my decision, but why?

Well I don't actually own BFA right now, and if I was going to buy BFA and start playing, after a couple months Shadowlands would be released and I would have to buy again, that shouldn't be a problem, if I didn't know that when Shadowlands is released BFA becomes free, well what's my point?Why blizzard doesn't make free upgrades for people that pre order shadowlands, since it will already be free when the expansion is released? And one more thing, why when we buy an expansion we don't earn at least a week of game time, it doesn't make sense to me that when you buy a game you actually can't play it if you don't buy a game time code for example, if I go right now and buy BFA, I won't even be able to play it??? What am I buying then?

I don't know it feels to me that this way of dealing with WoW Service is a little bit "obsolete" am I the only person that feels this way? Probably not, so think how many people want to play the game, but don't want to spend money with a feeling that they are getting "robbed" or something, I don't wanna be a fool am I wrong feeling this way? What do you guys think?

Edit1: I actually didn't know that you can't pre order shadowlands without BFA, but there's almost no difference, if I could just wait to buy on release date, it wouldn't be a pre order, but I would still get BFA for free and still wouldn't be able to play if I don't buy game time aswell

TL;DR Why doesn't blizzard give free upgrades to BFA for people who pre order Shadowlands? Why we don't earn game time when we buy an expansion? If I buy BFA right now and can't actually play it, it feels that I'm buying nothing, what do you guys think?