Ghost of tsushima sold for 60$ for almost the entirety of its first year and got sales in holidays to 40$. The game had a strong launch, was released polished and had with a score of 80+ on websites.
On the other hand, Days gone price went on sale for 40$ around 3 times during its first 6 months and was permanently dropped to 40$ for 2020. It was also frequently going on sale for 20$ during 2020. Getting launched as buggy as it did, didn't help it in any way. It had a 71 rating in metacritic and was bashed in most websites.
You can't compare both just based on unit sales. You have to take in consideration ROI, net profit and other things to make a direct comparison. As it stands right now with the information we currently have, the only reason days gone got to 8 million is because of the price drops. Dropping to 40$ and 20$ in the first 12 months doesn't look good to the public and investors and doesn't really help build up hype for a sequel.
Finally, do we even know when did days gone hit the 5 million milestone? Ghost of tsushima hit that milestone in Nov 2020. So many unknown variables needed to discuss the success of days gone compared to other PlayStation first parties.