What is the common denominator of all the lipids?
As you note, most biomolecules — or more precisely primary metabolites — exist as polymers that can theoretically be infinitely long and consist of well-defined monomers. This is true for nucleic acids and the building block nucleotides, for proteins made up of amino acids and for carbohydrates made up of monosaccharides.

Lipids, on the other hand, do not have that kind of polymeric possibility. In essence, they are trimers or monomers by definition, depending on how you look at them. Furthermore, all lipids contain the same (with same I mean identical) connective molecule namely glycerin or propan-1,2,3-triol $\ce{CH2OH-CHOH-CH2OH}$. Glycerin has three ‘sockets’, if you wish, namely its three hydroxy groups.

The ‘plugs’ that connect to glycerin always contain an acid functionality: typically this is either a carboxy group (fatty acids) or a phosphate (e.g. phophatidyl choline). The bond between ‘plug’ and ‘socket’ is an ester bond.

Both the fatty acids and the substituted phosphates can be structurally diverse. If you wanted, you could label the different fatty acids ‘monomers’ and the overall lipid molecule a ‘trimer’. However, the most typical difference between fatty acid residues is simply their length ($\ce{C16}$, $\ce{C18}$ or $\ce{C20}$ — typically multiples of 2) and their degree of unsaturation (e.g. stearate, oleate, linoleate and linolenate have increasing degrees of unsaturation). Attempting to teach students more than a few exemplary fatty acid residues or phophate residues is rather pointless since the choice of fatty acid hardly has any effect on the properties of the overall molecule, and aside from generally knowing what constitutes cellular membranes the same can in principle be said for the phosphates. Hence, students are hardly ever bothered.