Because a lot can happen in 10+ years. Assuming Ovi breaks the record (next year most likely -- about a 99% chance barring any catastrophic injuries) and gets 900 goals and stops Matthews would need to average over 50 goals a season for the next ten years (counting this upcoming one) to catch him.
That's ten years without significant injures, player strikes/lockouts, etc. Though really with that long of a time-frame it could just be cumulative effect of minor injuries that slow him (Matthews) down and he ends up missing. That length of time it's not even a given Matthews will get 700 goals (roughly double his current total).
Could Matthews do it? Absolutely, but talking about it right now just comes across to me as an annoying Matthews fanboi trying to steal some of Ovi's thunder. Really shouldn't talk about Matthews and the record until five years from now at least.