Reusing assets is common-place across a lot of industries. Example - most animated TV shows re-use the same animation scene over and over again, most games re-use the same body/head models/textures, design companies use the same assets in multiple designs, etc.

It's done to reduce costs and time involved in re-creating what effectively is the exact same thing they already have elsewhere.

I wouldn't say FFXIV has all completely original models and assets either - many of the enemies (including the models) can be found in previous FF games - some of the models are pretty much identical to their FFXI versions (just increased poly-counts and an improved texture).