Because the ת in בִּגְתָן doesn't get a דגש.
However the only reason ת in וָתֶרֶשׁ has no דגש is because it is preceded by a ו, just like the ב in וּבִנְיָמִֽן loses its דגש because of the ו which comes in front it the name Binyomin. בגדכפת in the beginning syllable or a word (as long as the preceding word doesn't end with an open syllable which connects to the word starting with בגדכפת) will get a דגש.
I wholeheartedly agree with you, and frankly, I'm a bit embarrassed that I didn't catch that myself. I like to think that I know my fair share of Dikduk, I'll be taking a refresher now...
FWIW I originally didn't realize you were coming from a Dikduk POV I thought it was just being pedantic