Live prices respond to every possession, injury, and momentum swing; books protect themselves with suspensions during scoring plays and reviews, and with delays on bet acceptance. For anyone consuming odds programmatically, stale live data is worse than no data - a price from twenty seconds ago may describe a different game.
ParlayAPI maintains sub-5-second freshness on major sports for exactly this reason, and offers streaming delivery (WebSocket and SSE) on the Business tier and up - see pricing and the live streaming demo. On why in-play is architecturally different from pre-game: live in-play odds.