There are 2 answers for this question and you already supplied 1 of them.
The designed products are not fully used by many designers. This is sometimes due to laziness on the part of the designer.
A designer that is alone who sets up a project in a project management application might be able to insert tasks, but actually they are not “using” the product and they are not experiencing the use cases that they designed the product for. Unless it has projects and team members that is real and interacts with it.
For products that rely on the data of the customer, you won’t be able to recognize what is hard or not clear unless you will be looking at your, relevant, personal data. Sample data is a poor substitute but it is better than nothing!
#2 is that, it is too late that the designers are brought in, in many organizations.
In spite of being included in discussions earlier where business goals and constraints can be identified, designers are usually thrown a written spec and they are told to “make it pretty”.
This is basically not possible: all of the design that is visual in the world can’t save features and workflows that havn’t been spec’d in the first place.