Why can’t they just offer more masses.
I wish I had more time to research the specifics, with citations, but I don’t. From what I remember from being a pre VII alter server is a priest could say 2 Masses on a Sunday or Holy Day of Obligation and only 1 Mass on a weekday, unless given a dispensation by his bishop. For example, where I grew up the parish had a pastor and an associate pastor, and weekday Masses were at 6:30 and 8:00 AM. If there was a funeral on a given day either the 6:30 AM would be canceled or the diocesan chancellery would need to be called to obtain the dispensation, which would be pro forma in our diocese (Spokane, WA) but the call still needed to be made in each circuмstance, a “standing dispensation” was not allowed in my understanding. A similar situation would exist if a parish had a regular Saturday morning Mass and there was to be Nuptial Mass later in the day. I recall reading that in some parishes (perhaps by diocesan decree?) nuptial and requiem Masses would usually be celebrated at the regular weekday or Saturday Mass, not as a separate event.
As all should know, Christmas and All Souls have three separate Mass propers and I’m thinking on these 2 days only a priest could say 3 Masses without a dispensation. However, I did see a quick reference that the All Souls day privilege was granted in 1758 by Pope Benedict XIV to the priests of the kingdoms of Spain and Portugal and the privilege does not apply to other territories.
All the above, which again is coming from my memory, was regulated by the 1917 Code of Cannon Law.
Today, under the 1983 Code of Canon Law, Canon 905 appears to permits a priest to celebrate 3 (or even 4, counting the Vigil) Masses per day. However, once per day is still the norm and the privilege to celebrate more often requires the permission of the Bishop. Here is an explanation of the current norm:
https://radioangelus.com/bishop-osei-bonsu-explains-how-many-times-a-priest-can-say-mass-in-a-day.
It would be interesting from a liturgical standpoint to see what protocols existed at the time of Trent, but I've not the time nor necessarily the resources to dig that deep. However, in tradition it has not been any individual priest's prerogative to just go say however many Masses whenever and wherever they like.
One notes that traditionalist communities may rely on a traveling priest for Mass, which is often said in the afternoon or evening, and that before Holy Communion they only observe a 3 hour fast from food, with water being permitted at any time. Until Pope Pius XII issued the Apostolic Constitution
Christus Dominus (January 6, 1953) Mass was not permitted after noon and from midnight a strict fast from everything
including water was required before receiving Holy Communion. As many traditionalist communities benefit greatly from these pastoral provisions of Pope Pius XII it is surprising how badly His Holiness is spoken of here.
On the Laws of Fasting and the Evening Mass