There are far too many cards for them all to be leagal and in tournaments with moxs and black loctus you can't be creative, you have to play the perfect deck and hope you go first.
I really think that those cards have gotten such a mystical rap that they're attributed to way more power than they actually had. I played in tourneys back when Magic was just starting to have tourneys, and they really weren't that significant. The problem is that every once in a great while, someone would get super luck and draw a couple mox and the black lotus in his initial hand and have sufficient other cards to be able to utilize the mana to do something nasty on the very first turn and everyone would be like "OMG! Did you see that!!!?". Then they'd conclude that it was unbeatable and unfair.
But they didn't make note of the 1000 other times the same cards were put into a deck in the same numbers and didn't result in a first turn boost like that.
But back then, there weren't any decks that pretty much guaranteed a kill within 4 turns and none that were capable of killing on the first turn.
Without banning or limiting cards, every deck in a T1 tournament would be capable of killing on the first turn which would mean that everyone would be forced to play blue for a set of Force Of Will's just so you can stop that because that's still about the only counter you can use without having any cards in play and lets face it, everyone being forced to play blue cards @#%^s up creativity quite a bit.
Back when they were made the P9
weren't as strong because while they made the game/your deck a lot faster, decks weren't that fast yet. By now decks are so much faster that these cards in full sets would just result in too many turn 1 kills where one player doesn't get to do anything.
That's just it, you don't end up with highly creative decks, or even thematic decks. You end up with decks that are nothing but overpowered and utility cards. Those aren't fun to play or to lose to, because they're just broken.
Except that the truly creative decks were the ones built to beat the decks made by guys who thought that just stacking their deck with powerful and utility cards would allow them to win. By banning cards you actually stifle the creativity. Honestly, it was usually super easy to wipe out the guys who spent tons of money buying lotus and mox and angels and thinking their deck was unbeatable because of this. Those guys usually didn't last through the first round because they were copying what the big strategy was last time around. But the creative players moved on. Similarly, the banned list followed what was the strong combos the last time around as well. All it really did was prevent the second string guys from copying last years super-combo (and likely losing).
The fact that you still seem to think of Serra's Angel as a powerful card shows just how awfully outdated your knowledge is. This is the kind of stuff people do with moxes and black lotus
And if those cards were allowed everywhere, decks such as this one
wouldn't be viable anymore because it simply can't compete with the speed that P9 cards give a deck.
And as far as creativity goes, try figuring out how this deck