There are a good few primary sources online, and more all the time, as material is digitised in one form or another. From the point of view of someone who works with translated material reasonably often (usually from manuscripts of medieval collections of recipes), there are two issues with translations.

The first is that translation is subjective - there are choices to be made when you translate even a single word from one language to another, because there isn't always one-to-one correlation between words. This gets more complex as you go into sentences, idioms, and phrases that require a local or historical understanding. To give a really simple example, there is an instruction often given in late medieval English recipes to 'seeth' something. 'Seethe' is a word which we understand to mean 'boil' in modern English, although we use it more in a metaphorical sense with regard to anger than we do the physical sense. But does that mean 'boil', or 'simmer'? It'll make a huge difference in the actual dish. Some documents use 'boyle', which is just an alternate spelling of 'boil', so can we assume then that 'seethe' is something else? (The Form of Cury, c. 1390, which I've linked in the sources below, uses both). Neither of these is an absolute, and the cook responsible for the recipe may not have been the one writing it down (literacy not being all that common among cooks), so we can't even be sure that 'seethe' was a proper choice. So to provide an actual translation, the translator needs to know enough about cookery in general, and specifically the cookery of the period, to know (or, really, guess) which is meant.

To give a more specific example of that, I've worked a lot from a book called Annals of the Caliphs' Kitchens, which is a translation with a great deal of extra commentary, glossaries, etc, of a 10th century Baghdadi cookbook referred to as 'al-Warraq', and known formally as 'Kitāb al-Ṭabīḵẖ' (which basically means 'cookery book'). The translator is Nawal Nasrallah, who really really knows her stuff.

There's a recipe there for a dish called Ṭabāhija, for which a part of the instructions go:

Slice 3 ratls (3 pounds) meat from a yearling lamb, wash it, and put it in a pot. Pour 1/4 ratl (1/2 cup) olive oil and 1/4 ratl (1/2 cup) rendered fat of sheep's tail, ad 2 dirhams (6 grams) salt. Sprinkle the meat with about 1/2 ratl (1 cup) water. It should be enough to cook the meat and then evaporate. So, when the meat is cooked and the pot is dry [...]

When I cooked this for the first time, I ended up with crispy slices of lamb. They were, in a word, weird. I had followed the instructions as precisely as possible, but there are references elsewhere to this dish being 'succulent', and what I had produced was not. It turns out that this is a Middle Eastern cooking technique called yahni, and the intention is that the heat is very, very low indeed, slowly roasting the meat as the oil evaporates. But even with a good translation, unless the translator added material (which they really shouldn't do) that meaning couldn't carry across because I was lacking some local knowledge.

The second issue, if that wasn't complex enough already, is that sometimes, we're just not sure what a given word means. I've seen this is in the last few months in Anglo-Saxon, in Greek, and in Irish. The Anglo-Saxon example was eþel. From context, it's reckoned it means 'home'. But it might mean 'land', or 'nation', or 'house', or even 'den' (as of an animal). There are undoubtedly other examples where there's just a single instance of a word in surviving writing, and since it's an isolated noun, it could be almost anything.

So the reason that there aren't more translations of period sources easily available is that translation is really difficult. There will be more of them over time, but they're likely to be books on which the translator has worked for years, rather than online resources which can be provided quickly.

Sources:

Nicholas Howe, Writing the Map of Anglo-Saxon England: Essays in Cultural Geography (2008)

Nawal Nasrallah, Annals of the Caliphs' Kitchens (2010)

Samuel Pegge, The Form of Cury (c. 1390)

I know Fordham had an online collection of different translated primary sources and Yale has a few too.

Is there any particular period/geographical area you're interested in?