I think the reason that the ISU does not give an extra bonus to triple jumps in combination is that there is already a big bonus automatically built in. Namely, you get a whole extra jumping pass. Here is a simplified example. Skater A scores as many points in six passes as Skater B scores in seven.

Skater A

3Lz+2T
3Lz
3F
3Lo
3S
3T
2A

Skater B

3Lz+3T
3Lz
3F
3Lo
3S
2A+2T
7th pass