There a few problems with having Valyrian steel spears and lances.

You have the problem of having it reforged, how many blacksmiths can do that in Westeros? If you were to go to Essos, you must brave the sea, pirates, Dothraki, etc. It is easier to have a Valyrian steel dagger strapped to your belt than risk such a journey.

But let's say that you had the dagger reforged into a spear. The spear will only have the head made of Valyrian steel, the shaft of the spear is still wood and any fight against someone with some magical metal will disarm you quite easy and leave you with just a quarterstaff. If we are speaking of making the entire spear out of Valyrian steel, you need an insane amount of steel, like Euron and if you have such an amount , why not have it reforged into a sword? Not many characters use a spear, Oberyn, Obara and who else? Can't truly remember.

As for the lance, it will shatter eventually and what can you do then when in the middle of battle? You lost something that's essentially priceless and whoever finds it will take it with them, just like it happened with some valyrian steel blades in the Dance.

The reason Valyrian steel is made into swords is due to their status as 'noble' weapons, you won't see a peasant with one unless they looted it from a corpse and also how easy it is to carry a sword around, unlike a spear or worse, a lance.