It can be difficult recruiting some sorts of support directly. Not impossible, but difficult.
If it's someone you know or for whom you have a personal recommendation, that's one thing, but if not, who takes responsibility for vetting them? Who is liable if something goes amiss, e.g. the PA leaves the front door open and the house is burgled?
That being said, I'd be wary of trusting an agency anyway.
When I wanted carers for Dad, I went through a reputable agency. I was seeing him every day, and also taking him to appointments, running errands for him etc. I arranged for someone to go in at lunch time. For a while before he went into a care home, he was having three visits a day.
But I began to get niggly because, for instance, despite repeated requests, they were rigid in some things and kept mopping floors day after day. They kept vacuuming little bits of carpet. I said just vacuum the middle one day and the edges another or whatever, but no, the agency had this rigid list.
After I moved back into the family home, I discovered the oven wasn't working properly. How many Wiltshire Farm Foods fish and chips ready meals had he been given with nearly raw fish?
But if I'd recruited direct, would I have done a better job? I'll never know.