They just don't have too many to choose from unlike in real golf. They are slowly but surely rolling out new pins for each course. They have said that it takes at least 4 months to do a new set, and with other stuff they have to work on (or choose to work on in some cases) they are hard pressed to do more than 2 courses a year.
They put out a new set for Chambers last year and have already put out new pins for PGA National this year, and will be releasing a new set for another course in one of the upcoming months.
Also remember that based on the type of game you play, you may get the same pins most of the time whether there are multiple ones available or not. Some games such as solo ranked, conditions including pin placement, are based on your Tier and not editable.