We all know that these additions to Bitcoin Core have slowed down transactions and made transactions more expensive.
Ordinals are not an addition to Bitcoin Core. People just take advantage of Bitcoin allowing arbitrary data being embedded in transactions.
What is the justification for them?
What kind of justification would be sufficient to you? They are valid transactions, and therefore are allowed by the protocol.
Why are they being added when they make the network less efficient?
They don't make the network less efficient. They just increase the fees. That's expected to happen either way sooner or later to sustain the miners' income.