Because (and I have to generalize here since this is a broad topic) partnerships can suck up a huge amount of time, with the following downsides:

1) you get introduced to customers indirectly (the partner's users), this isn't necessarily a bad thing but you'd rather have customers come directly to you.

2) the partner's objectives as a business are not your objectives, they may demand exclusivity of you or prevent you from doing business with their (the partner's) competitors. This may come to bite you if your startup grows, evolves or pivots and a better partnership comes along.

3) partnerships usually involve contracts which cost time and money.