Why? Because people do it.
Here's my non-linguistics take:
Vowels are voiced. The difference between t and d is the voicing. If you don't stop voicing after the vowel and let it slide right into the next vowel, you change your t to a d along the way. If you have to "unvoice" for the next consonant, then your t remains a t.
The "oo" sound can actually like y+ew, so the second t sounds like a t before that y.
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