I’ll try to provide some answers to this by looking at the context, what Leonardo actually did (or did not) finish, a particular case (of a finished work that took decades to deliver), and what patrons might have expected from him.

First, Italy is notorious for unfinished works of art (painting, sculpture, and architecture). And “unfinished” in this context could mean a lot of things: some things were just in a very drawn out state of completion—e.g., Florence Cathedral went decades without its dome. After construction on the current building was started in 1296 it wasn’t until around 1450—a century and a half!—that the dome was completed, and not until 1469 that the dome was capped with its copper ball. And at that point the façade was still unfinished! For the façade you’d have to wait until the 19th century…

Just a few minutes by foot from Florence Cathedral is San Lorenzo, a church that *still* lacks a proper façade. There was a massive project to finish the façade that Michelangelo started c. 1515 but it never moved beyond the planning and quarrying stages.

Or consider Orsanmichele, the granary-turned-shrine in Florence. The Silk Workers’ guild planned for collaborative decoration of the exterior in 1339 by the city’s major guilds—but by 1406, the city government had to ask that guilds finish their projects within ten years or forfeit their rights to the tabernacles (external niches) they’d been assigned. This did inspire work to pick up again, but ten years later work on these sculptures—by some of the most important figures of the early 15th century—was still ongoing.

I could go on, but this should convey my point: unfinished works, or works in a long state of execution, were not necessarily rare in Italy. (And given the timeframe for projects like the Metro C line in Rome one could easily argue this is still the case; but I digress…).

That said, Leonardo did make a habit of leaving things incomplete. But lest we forget to give him credit for the works he *did* finish, consider the following (partial) list of finished works that are generally accepted to be mostly or entirely by Leonardo:

The Annunciation (c. 1476)

Madonna of the Carnation (c. 1470s)

Portrait of Ginevra de’ Benci (c. 1480)

Benois Madonna (c. 1480)

Virgin of the Rocks (two versions, late 15th century to early 16th—it’s complicated, but I'll get to it)

Lady with an Ermine (c. 1490)

The Last Supper (c. 1495)

Virgin and Child with St Anne (c. 1515)

Mona Lisa (c. 1515)

St John the Baptist (c. 1515)

That’s not a bad run, especially given his time devoted to so many other pursuits. Admittedly, a lot of his other pursuits were also unfinished (e.g., various planned treatises, the bronze equestrian statue to Francesco Sforza) but this wasn't always his fault. Works by artists are left unfinished for all sorts of reasons—money runs out, artists die, patrons lose interest, patrons switch interests, cities get invaded, etc. So as a bit of an aside, remember that "unfinished" doesn't always just mean an artist simply abandoned a work in search of different or better work.

All this being said, Leonardo did leave a lot of stuff unfinished. And in some cases this likely did frustrate his patrons. One of the better documented cases concerns the Virgin of the Rocks (which, despite being finished in not one but two copies, had a very drawn out execution). The Confraternity of the Immaculate Conception in Milan commissioned this painting from Leonardo and the brothers Evangelista and Giovanni Ambrogio de’ Predis in April of 1483 (per a surviving contract). It was apparently due in December of that year, which is just laughable. There are records of payments for a couple years, and eventually the first version of the painting (which seems to be finished by the early 1490s and is now in the Louvre) is at the center of a dispute (with lots of documentation) over, among other things, payment and fair judgment of the work. It's a tangled history, but it seems Leo and his pals felt the work was worth more. Ultimately, Duke Ludovico Sforza seems to have taken the picture—commandeered it, lets say—to give as a gift, resulting in more legal wrangling over the execution and delivery of a second version of the painting (completed about 1508 and now in London’s National Gallery).

So yes, on at least this occasion (and likely others), people were frustrated with paying for something that was not delivered. But in this case at least they got something, even if it took about 25 years. There are plenty of cases where the patron got nothing.

As a final point as to why people might have tolerated this, consider that Leonardo’s status by his later years was such that people were keen to have anything by him. Isabella d’Este, Marchioness of Mantua and a notable patron of the arts, wrote numerous letters trying to get either a portrait of herself by Leonardo or—failing that—*any* painting by Leonardo. And she wasn’t alone—for part of the time that Isabella was writing to ask about Leonardo doing her portrait he was busy painting a work for Florimond Robertet, a secretary to the King of France; subsequently, this inspired the King himself to want a painting by Leonardo! Whatever the potential frustrations of dealing with Leonardo, patrons of the highest order felt it was worth it.

The literature on Leonardo is vast, but if you're looking for accessible sources on him and his works then books by Martin Kemp are a very good place to start.

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EDIT: typo