64GB is also the limit of Intels desktop i7 processors. I don't think Ryzen is intended as server processors, so 64GB should cover the target market very well.
Broadwell-E i7's now officially support 128gb.
All the Xeon equivalents (e5-1650/60,80, etc) support 256-768gb depending on the model, if paired with a consumer board that limit is usually 64-128gb.