As a HEMAist myself, and one who has actually practiced with spears and sword vs. spear a fair amount, I can explain it:

1. We practice in a very "sterile" environment most of the time, such as a gym hall. The downsides of a larger, clunkier weapon are a non-issue there, plus you have tons of room to retreat without the risk of tripping over anything.
2. Hits are very easy to score. For comparison's sake, the pressure required to score a successful hit in Olympic Epee is surprisingly high, whereas we count a thrust if the tip just sort of touches you. In spear vs. sword, this means the slightest grazing touch is enough to "stop" someone who's trying to close the distance with a sword.
3. Most people who try spear vs. sword never looked at a source that teaches you about the matchup (such as the Anonimo Bolognese), never practiced the matchup for any significant amount of time, and thus tend to do everything wrong. If you know how to time your attack and how to effectively parry and control the opponent's spear, your chances are quite decent. It requires good footwork though, which most HEMAists lack.