Because forming the seal takes some years. Back in Part 1 Tsunade said to Orochimaru how it took her some time to form it, but it didn't say how much. Now Kishi answered the question. 3 years. And as proof it takes some time is the fact that Orochimaru didn't knew about the seal which is only normal to assume the fact that indeed it took Tsunade as well 3 years to form her seal given that prior to that Orochimaru didn't knew about it.
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And another proof to it, it's Shizune who states about the requirements of the seal to be formed and since she is Tsunade's student, there you have the answer.
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