The answer to this is several-fold.

1. While half-swording uses a sword purely to thrust, this isn't really the same thing as a spear. While a lot of the effectiveness of a spear (and especially a pike) is its reach, half-swording does not produce reach. Instead, it produces very fine point control, in order to better enable the swordsmen to thrust through gaps in armour. Thrusts in this case are not about punching -through- full plate armour (which is nearly impossible with a sword) but about going -around- it. The short length of the sword in the half-swording position allows for that fine point control - a full length spear, let alone a pike, wouldn't have that level of control.

2. There -were- dedicated thrusting weapons designed to be used in a manner similar to half-swording. The Estoc was a dedicated thrusting sword that effectively had no cutting edge. It was a specialist weapon for attacking armoured opponents. As such, it wasn't as versatile as a sword, which leads us to...

3. Half-swording is a way to use a sword, which is a very 'all around' weapon, in a specialized way. In the age of full plate armour (15th and 16th centuries) the sword wouldn't be the primary weapon of a fully armoured man at arms except in judicial or 'sporting' foot combats where the rules specified swords. When mounted, this would be a lance, and when dismounted, this would be a pollaxe or other polearm (in the 14th century, men at arms in England and France might fight with a 'short lance' or spear when dismounted). However, this didn't mean that swords were useless and unused sidearms. Instead, they were versatile weapons that could be used against a wide variety of opponents (armoured and unarmoured) in a variety of different ways (half-swording against armoured opponents, cutting down unarmored opponents, even couched like a lance when mounted!). By giving men at arms the best chance to thrust through gaps in armour, half-swording was a technique that made a versatile weapon even more versatile.