Tl;dr: I don’t think there is a simple answer!
As a few people have said in various ways, there are several issues:
- There is no universally accepted definition of a species. Groupings above that (genera, families) become even more artificial.
- Some people expect that a given amount of morphological variability is to be expected within a species (look at humans!). Others (someone already brought in splitters) will make the argument that if something is distinctive enough to be referred to as an entity, we need a name for it, and often describe these as new species.
- Some taxonomists don’t believe in infra-specific taxa: subspecies, varieties, forms, and so they describe everything as new species. Others (myself included) think that infra-specific taxa can be used effectively to explain the variation we observe in the plant world.
- Unlike a majority of animals, plant hybrids are common and often perfectly viable, sometimes able to back-cross with the parent species. This allows for gene-flow even after populations have differentiated past the point where we’d consider them different species.
- Plants don’t read taxonomy books and consequently don’t realise that we expect them to fit into these neat little named boxes we’ve created for them.
- Unlike the situation with arthropods and fungi, where taxonomists barely keep up making sense of the vast number of unknown species, vascular plants at least are getting fairly well known. This leaves more time to argue about where one species/genus/family ends and the next one starts, and to refine the taxonomy to better reflect the evolutionary history of taxa.
- Some groups are just hard. We keep expecting that if we throw the next whizz-bang new technology at the hard groups, they will magically resolve.
I hope these points make sense!