An account is a ledger entry

Strip the wallet UI away and a Stellar account is one row in the replicated ledger: a public key, an XLM balance, a few flags — and the sequence number you met while dissecting envelopes (the replay-proof counter).

Rows are not free. Every validator stores every entry, so each entry must lock a base reserve of XLM — currently 0.5 XLM, with a fresh account holding at least two (1 XLM) it cannot spend. Delete entries and the reserve comes back.

The reserve is not a fee. It is rent-by-deposit: the ledger stays lean because bloat has a price tag.