What "Everyone" says isn't true!
A related issue was discussed on this forum recently ("If NoSQL stands for "Not only SQL", is SQL a subset of NoSQL?"), and in my answer, I mention 2 KV systems, one of which (RocksDB) has a subset of ACID transaction characteristics and one of which has them ALL (FoundationDB).
BTW, MongoDB is NOT fully ACID compliant - see the report from the highly respected JESPEN site here. There is further reading to be done here and here - but there is nothing to stop a system which fully supports SQL from being BASE and vice-versa...
And remember, don't believe everything a vendor tells you - they (ahem...) may be biased.
Also, don't forget that BASE is a perfectly valid strategy - for the likes of Amazon and/or Facebook where the odd cancelled order and/or lost post isn't going to be the end of the world - however, banks cannot (afford) to think like that - their systems have to be ACID - at least the ones handling financial transactions - maybe they perform their customer loyalty programmmes on a BASE system - lower machine specs and no requirement to maintain consensus/correctness leads to lower costs!