CP1252 grew out of a combination of ECMA-94 (which was derived from DEC's MCS) and Apple's Mac Roman (*1), these 5 positions are reserved for future expansion, a future that never happened. (*2)
It was common back then to have positions reserved for future extension (*3). Spare codepoints would be held for important additions otherwise not possible - like it was with assigning the Euro symbol in the late 1990's (*4). Filling them with random letters just to use them would be a short sighted waste of this extreme limited resource.
A single code page is not really able to fit all Latin based language,not even the European ones. Adding 5 more characters would not be enough to make even a single additional language work. Even more as that issue was already resolved by using language specific code pages.
It might also be not very likely that they will ever be filled, as there are other ways in use since more than a decade. CP1252 is a relic for compatibility.
Some background/timeline to Paul Humphrey's mentioning of Office as base (and as usual way too long for a comment)
While MS Office wasn't a thing until August 1989, MS Word was.
Microsoft Word was published for DOS in 1983 using its own character set, based on DEC's MCS.
MCS is a precursor for the 1985 ECMA-94 character set, which in turn was adopted by ISO as 8859-1 in 1987, so way after the definition of CP-1252
Word was ported early on to the Mac, published in spring of 1985.
Considering this it's natural that Windows 1.0, published in November 1985 uses a charset that supports their major publishing product Word.
Though, Word wasn't sold with Windows 1.0 as Word, but as Microsoft Write - this was most likely due a deal between Gates and Sculley (*5).
All symbols found in at rows 8x/9x are either to be found prior in the DOS-Word character set or the original (*1) Mac Roman character set.
22 of the 27 characters in row 8x/9x of today's CP1252 are neither present in 8859-1 nor CP437 nor CP850, but in Mac Roman.
Most likely the remaining 5, are later additions. As Another-Dave mentions as well. The 1996 introduced Euro symbol for sure is, as user3840170 notes.
- It wasn't until 1989 that an actual Word, based on a then up to date Mac Version, was released for Windows 2.11.(*6)
*1 - The original Mac Roman charset had only 237 characters and did grow from there on. Most additions happened until Mac OS 6, the Euro symbol for sure later.
*2 - So far :))
*3 - At least for somewhat forward looking designs.
*4 - In fact, the Euro symbol is a great argument for keeping some spare characters, as ISO's attempt to reform 8859-1 as ISO-IR-204 to add it simply didn't work out.
*5 - Sculley threatened Gates to stop delivery of Windows 1.0 due being too similar to the Mac, which Gates countered by pulling WORD and EXCEL for the Mac, which were the leading products in their categories. The agreement lead to Word being 'not published' for Windows.
*6 - Obviously felt Gates no longer bound by that agreement as Apple sued Microsoft anyway for Windows 2.0 in 1988 .