In this article about the sharded blockchains, (Link to article) we read:
The core idea in sharded blockchains is that most participants operating or using the network cannot validate blocks in all the shards. As such, whenever any participant needs to interact with a particular shard they generally cannot download and validate the entire history of the shard.
What reason does cause that a validator in shard A cannot download and validate the correctness of the entire history of the shard B ?
From my point of view, that validator is able to download the entire historical transactions in any arbitrary shard (exactly similar to a non-sharded blockchain) and then verify the history of all transactions by tracing each transaction.
What is the problem? (Where typically all the shards use the same consensus algorithms and all of them have the same block structure.)