I noticed that the principal forms of verbs always only include the neuter participle form, e.g.
vocare - voco, vocavi, vocatum
Is there a reason I've never seen the following?
vocare - voco, vocavi, vocatus
Thanks for your answers.
The ultimate answer to these sorts of things is always "convention." They do it because the what they worked with did it. Maybe some editor or another justified their particular adoption of it, but I'm sure that's a rarity.
However, in this case, the answer is actually "you just haven't seen them yet."
There are dictionaries which use the masculine ending of the perfect passive participle in their dictionary endings. Bantam chiefly comes to mind, but also several glossaries in primers/introductions, including Mondon, Shelmderdine, Moreland & Fleischer, and Ecce Romani. I'm sure plenty more do, too, but these are the ones I have next to me.
Every Latin verb has three stems: the present stem, the perfect stem and the supine stem. The dictionary is tasked with telling you, for every verb, what the three stems are. Therefore it gives you (usually in abbreviated form) the first person singular indicative active of the present and perfect tense and the first supine (a.k.a. the -um supine).
The third form is therefore not the perfect participle (although it shows you how to form the perfect participle). You can tell, because the same dictionaries, often enough, have occasion to name the perfect participle, and in that case they use the masculine form. This happens when the verb has no perfect active forms, either because it is a semi-deponent, or because it is a deponent verb.
Consider, for example, the Lewis & Short entry for audeo:
audĕo, ausus, 2, v. a. and n.
Or for gradior:
grădĭor, gressus, 3, I. v. dep. n.
Admittedly, yes, the supine stem could also be called the perfect passive stem, and there are grammarians who do that. And of course, whatever you call it, you could also give any form you like of the perfect passive to indicate it. (Maybe a feminist Latinist will soon publish a dictionary that lists laudo, avi, ata. At least the usability would not be impeded.) But I think the signs are strong that the form usually listed is supposed to be the supine.