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because the rulebook says 302.6. A creature's activated ability with the tap symbol or the untap symbol in its activation cost can't be activated unless the creature has been under its controller's control continuously since his or her most recent turn began. A creature can't attack unless it has been under its controller's control continuously since his or her most recent turn began. This rule is informally called the "summoning sickness" rule.
And I was thinking that if I flash it in on my opponent's turn, I haven't controlled it since my most recent turn.
If you flash a creature in at the end of an opponent's turn, you then have an untap step, then an upkeep, then a main phase. In that main phase if you want to activate abilities of that creature, or in the following attack phase you want to attack with it, it says "Did you control it since the start of this turn?" to which the answer is "yes"
You can think of it this way: A creature has summoning sickness whenever it comes into play/changes controllers until the beginning of its controller's next turn. That's why cards like Act of Treason HAVE to give the creature haste in order to be useful, or all creatures taken from an opponent in this way would have summoning sickness for that turn, and then be given back.
Flashing in a creature at the end of an opponent's turn is a pretty effective way of getting around this while keeping your creature out of danger from sorcery-speed dangers.