Church building maintenance is a huge portion of most dioceses' expenses, and with dwindling parishes and lack of funds, they simply can't keep all of them open. Some of these beautiful buildings cost more in maintenance each year than it would be to build a newer, more modern one that was sized correctly for the current number of parishioners.
Much of Europe has already gone through this; many (but not all) of the churches you can visit in Europe have significant state support because they're historical and touristic.
Sometimes the diocese gets enough people interested in "saving" an old church that they can cover the maintenance for another year, decade, etc, but we're talking millions of dollars.
Rome supplies some funds, but not much and they're almost always used for missionary work or charity, etc. Peter's Pence is one example: https://www.vatican.va/roman_curia/secretariat_state/obolo_s...
The Catholic Church appears as one big institution run by an absolute monarch, but it is much more like a fiefdom run by local bishops with a king who can if he really works at it, depose one. And the parishes under the bishops are similarly somewhat independent. The canon laws covering it can be seen here: https://canonlaw.ninja/?nums=1254-1310 (note that the bishop "taxes" the parishes to support the diocesan activities, this is usually done as an "annual Catholic appeal" which transfers money from the rich parishes to the poor parishes, usually the ones with schools). A parish financial committee is established by https://canonlaw.ninja/?nums=537 but the pastor has some pretty powerful leeway in executing his duty as he sees fit.
Even many Catholics don't realize that their favorite "old parish church" is often kept afloat by donations from one or two old parishioners. The old couple handing out donuts after Mass may be donating a million a year to keep the parish school running, and you may only ever find out if you dig into the non-profit filings. As an example, a local (well regarded) private Catholic school K-12 has $4m in tuition income, and $2m in donations, with the rest from "other" which includes selling tickets to games, merchandise, etc. Of that $7m, almost $4m goes to salaries, and about $1m to building and maintenance.
Find your local parish and search for "annual report" and you can see more.