That's because in the EDA space, they have horrible software practices. Take the worst you can imagine and then go worse from there. So eventually, when things "appear to work" they end up taking a snapshot of a messy directory and calling that the deliverable asset.
No, it's just all of the device models. Even the OSS tools take up almost 20 GB of space once you get all of the suppported device models which is not many compared to what Vivado supports.
Why not both? Look at the installables from Cadence, they are full of redundant garbage, broken rpaths, etc. Tools that don't even need to support devices have 5-8GB compressed installables with 12-20GB of on disk space being taken up.
Horribly broken installers and a mess of environment variables needed to get the software to even run.
