Interesting question. Here's how to know for certain that abstract nouns are always singular, despite their frequently plural forms. As given:
A noun denoting an idea, quality, or state rather than a concrete object, e.g. truth, danger, happiness. [Oxford]
- If an abstract noun denotes an idea, it is singular because "an idea" is singular.
- If an abstract noun denotes a quality, it is singular because "a quality" is singular.
- If an abstract noun denotes a state, it is singular because "a state" is singular.
- If an abstract noun denotes a concept, it is singular because "a concept" is singular.
And so on. Then, there is the fact that some nouns do not have any singular form. Example: trousers. "I'm hanging up my trousers." may mean that you are only hanging up a single garment.
The abstract nouns whose singularity you question, just coincidentally happen to fall into the category of nouns which must be posed in plural form, in order to convey their meanings in appropriate context.
So being in plural form doesn't necessarily mean that those particular abstract nouns are plural, in the sense that they represent more than one idea. Rather, each one represents just a single concept.
There are also many abstract nouns which are not posed in plural form: childhood, stewardship, direction, forgiveness, etc. -- as with concrete nouns.
So in conclusion, there are singular nouns which are expressed only in plural form. And, all abstract nouns, regardless of form, are singular.
(Singular and Plural Nouns)
(Cambridge author Peter Lucantoni in a training video on abstract nouns...)
UPDATE (correction): How embarrassing, I was wrong:
Some abstract nouns can be used uncountably or countably. The uncountable use has a more general meaning. The countable use has a more particular meaning.
Nouns of this type include: education, experience, hatred, help, knowledge, life, love, sleep, time, understanding.