I’m thinking the reason is likely one of logistics and convenience. Sir Alec Guinness is dead, so they’d probably have to go scrubbing through the original trilogy audio to find the lines they’d need. It’s probably easier and quicker for them to just write a quick line or 2 and then pay Ewan McGregor to come in and record.
Within the ST, why does Obi-Wan’s voice sound like Ewan McGregor’s? The last we see of Obi-Wan is at the end of ROTJ, where he’s shown as Alec Guinness, so isn’t that who he should sound like? Unless you changed the ending to where his Force Ghost is Ewan McGregor standing next to Hayden Christensen, it doesn’t make sense.
I understand that the parts where it’s Alec Guinness saying “Rey” were taken from archival footage, so that’s probably the reason why.
Analog Releases of Films That Contain Deleted, Extended, & Alternate Footage That’ve Never Been Released on DVD/BluRay