Why do they put the jelly-like substance in pork pies?
There used to be a good reason to add the jelly to the meat pie: food safety. In the time before refrigerators, it was hard to keep meat without some spoilage. But a slaughtered fully grown pig meant some hundred kilos of meat, and it wasn't eaten on a single day. 

Most of the bacteria which spoil meat need oxygen to proliferate. So once you pack the meat into a clinging skin, it keeps for longer. This is one of the reasons why people bothered to bake meat pies instead of roasts in the first place. 

But there is a problem with meat pies. As the other answers mentioned, the meat steams while being baked, and this steam must be gathered below the upper crust and vented through a hole. You can't tightly wrap the meat in the crust and then bake; the steam will probably open the seam, resulting in an irregularly shaped pie, and the crust still won't cling. So, a meat pie has some space between the meat and the roof. 

I don't know how quickly such a pie will dry out, as ElendilTheTall suggested. Surely, this is a factor. But I bet that, if you keep it outside of a refrigerator, it will spoil long before it dries. Filling this space with jelly (which happens to be available in big amounts too - after all, we just slaughtered our big pig and want to cook lots of it as quickly as possible, so we probably have more stock than we can use up) practically seals the meat airtight against bacteria. And while the cooks from that time didn't know about bacteria, they sure knew how quick a piece of meat spoils visibly (smellably?). This is how the traditional jelly-topped meat pie recipe was born. 

We have refrigerators today, but we still follow the recipes as they always were. I don't see any reason not to. Drying out is probably a factor. And as for the taste - I have eaten more French patês than English meat pies, and maybe there is some difference. They never looked like on sarge_smith's picture. But I definitely like the jelly layer. I must confess that I have always eaten it in good restaurants or home made, so maybe the poor quality has ruined it for you. 

But anyway, if you want to bake meat pies without it, you don't have to include it. The problem is that, if you leave the space hollow, you'll have a cosmetic problem (your crust will probably shatter when you try to cut it) and the already mentioned drying possibility. The solution is to bake the pie without the upper crust. You'll then have meat pie slices which only have crust on three sides. If you don't want a baked crust to form on the meat, or if you experience heat control problems because of the missing insulator, use a temporary cover (aluminum foil, bacon stripes, or lay some big lettuce leaves on it and throw out later, or you can try a plate, but must somehow leave an opening for the steam). If you want a pie with four crust sides, blind bake a sheet of pastry pre-cut for the open side (allow for shrinkage when cutting), then glue it to the meat pie somehow. Sticky honey glaze, or a layer of cream cheese should work (as would jellied stock :) )

This doesn't guarantee that you won't get some congealed fluid within the pie. sarge_smith correctly pointed out that the meat in a pie is collagen-rich, and all the juices which would become roast drippings in roasted meat are staying between the meat and the crust. Some will get absorbed, but maybe not all. It may be worth to try making the pie with ground tender meat. I am however not sure whether this will provide you with a good jelliless pie, or with a pie with a soggy crust.