To understand ‘why’ you have to step outside your particular use case and look at the fact that Infusionsoft had to account for the needs of all users. Meaning that there are many that have need for more than one active opportunity to be used per contact. So then the question is, how do you use merge codes and how do you identify in campaign builder, which opportunity record to use?
We’ve also solved for this for clients by providing ways to identify them using naming conventions that make them uniquely addressable but out of the box, there isn’t a way in campaign builder (which is contact-centric) to know which opp record to use.
While it looks like Zapier accesses opp records, it doesn’t look like they can grab results of webforms directly, and I don’t see a way to get the updated contacts custom fields to use either.
This explains why we’ve had so many ask us for a solution (unless I’m missing some aspect of Zapier but I looked and didn’t see anything that would be able to do it). They would need an ‘update’ trigger and it looks like they only offer triggers for newly created records.
Additionally, if your use case does actually involve multiple opp records an single contacts, then Zapier will have no way of knowing which opp record to work with either.