But what's the point to use these DIY object storage systems, when they do not provide durability and other important guarantees provided by S3 and GCS?

When you want just the API for compatibility, I guess?

Self-hosted S3 clones with actual durability guarantees exist, but the only properly engineered open source choices are Ceph + radosgw (single-region, though) or Garage (global replication based on last-writer-wins CRDS conflict resolution).