Because every view of our data and every field is treated as a special case, and basic capabilities (sort, filter, search, save and share multiple views) are doled out with great parsimony on a field-by-field and view-by-view basis. I do see some evidence that the data architects at Asana have gotten the message that their data models and constructs need to be modernized and made generalizable, but they are finding it to be a daunting task.
BTW, data that you store in custom fields is not searchable within Asana at the moment. You would have to go to a view of a single project that uses the custom field of interest (though custom fields are global to a workspace) and use your browser search tool to find data within a custom field. If you’re not sure which project that data resides in, you’d have to open likely candidates one by one.