Bitcoin Core internal Electrum Server Is there a technical reason for which there isn't an option in Bitcoin Core that would deploy an internal Electrum Server, or something similar so that a software wallet can fetch addresses and balances?
Electrum servers are simply not compatible with Bitcoin Core on a development or operational level. They require a vast amount of indexes created which the node does not have, and must be created by an external tool. With the state of the art electrs Electrum server, it needs upwards of a day of time to create these indexes, and more than 50GB of additional data stored on disk. For the server to be performant, forks of the software use over 1TB of additional storage for these indexes.