I've worked on some titles very similar to Yakuza and I keep espousing the way they develop their titles. I don't know for sure because I haven't worked at the studio but it really seems like they've streamlined their development to keep costs down and allow them to develop quickly via asset reuse and resisting major engine upgrades across titles in favor of a slower, more focused pace.

Often in my experience in the west we tend to re-author assets, do major engine upgrades or re-implement gameplay systems across sequels when we could have iterated on existing systems and use the time we saved to work on new stuff.