I think your question can be answered in two ways,
The shorter explanation:
I believe that they are/were independent projects and there was no centrally coordinated goal.They were separated in their own sense when they were first introduced but as they evolved comparison and overlap seems to have been a case.
The detailed explanation:
Some of the comparison points are given below:
Apache Pig may be a scripting language and Hive may be a SQL like search language.
Hive requires very few lines of code when compared to Pig because of its SQL like resemblance.
Pig has issues in handling unstructured information like pictures, videos, audio, text that's unequivocally delimited, log data, etc.
Pig is faster in the data import but slower in actual execution to a language like Hive.
Pig has no metadata support, (or it is optional, in future it may integrate hcatalog). Hive has tables' metadata stored in database.
So to conclude, the purpose of both are different but under the hood, both do the same, convert to map reduce programs.