Good questions
@Bit of hope as I have also worked in the area of passport fraud so have a bit of experience with fake IDs (though once again not Mexican):
There are different types of ID frauds:
(a) totally fake IDs - a very old-style fraud that pre-dates computers and easily-checked databases. Even in developing countries this type of fraud is being replaced by more sophisticated ones.
(b) legitimate ID obtained using fake personal details - the 'next phase' that replaced (a). Still common in places where there is no central database to check people's IDs - for example in countries where there is no centralised registry of births, or where you can obtain fake 'base' ID documents quite easily.
(c) imposter fraud - use someone else's documents to obtain an ID with your photo on it. For example Johnny has a criminal history in the US. He is deported back home and then uses his brother Sammy's documents to obtain a clean set of documents. Variation on this theme is simply to borrow someone's ID and use it (particularly if Johnny and Sammy look alike, Johnny may just use his brother's photo as his own). Note: one of the reasons countries now record biometrics such as fingerprints, face scans etc for visa applicants and deportees.
(d) tombstoning - obtain the details of a person who is deceased, get their documents and obtain ID in their name. Children are preferred because there will not be any adult ID photos or fingerprints on file, no will they have a criminal record. In many countries the historical death records are not yet linked to new ID systems so may not flag this kind of fraud.
As to the ease of obtaining, obviously that can vary by location, local and national preventative systems, LE capacity/willingness. In many communities it will also be seen as a legitimate 'means to an end' whereas in others it would be a big no-no.