Try as I might, I didn't find any source that told me why these were called the Monuments of Cycling. Common sense would say they are the most grueling and steeped in lore 1 day races on the calendar.
In the process of my search, I came across this cycling quiz from the Daily Peloton which contained the question:
41) - Name the riders who have won all of the five monuments in cycling (Paris-Roubaix, Milan-San Remo, Tour of Flanders, Liege-Bastogne-Liege, Tour of Lombardy).
The answer is: Eddie Merckx, Roger De Vlaeminck and Rik Van Looy. Van Looy was the first to do so. Merckx was the only one to have won all 5 multiple times. Is there anything he didn't do??
Others have come close- Sean Kelly won 4 of them, but only managed a 2nd place Ronde van Vlaanderen in 1984.
Milan San Remo
Merckx 7X: 66, 67 69 71, 72, 75 76. Seven, here say it again for effect: seevvvven.
Van Looy 1X: 58
De Vlaeminck 3X: 73 78 79
Merckx 2X: 69, 75
Van Looy 2X: 59, 62
De Vlaeminck 1X: 77
Merckx 3X: 68, 70, 73
Van Looy 3X: 61, 62 65
De Vlaeminck 4X: 72, 74, 75 77
Liege Bastogne Liege
Merckx 5X: 69, 71, 72, 73, 75
Van 1X: 61
De Vlaeminck 1X: 70
Merckx 2X: 71, 72
Van 1X: 59
De Vlaeminck 2X: 74, 76
Sources: wikipedia, Daily Peloton and this velo archive site.
Here are the answers to the quiz.