I have hard pruned a bunch of older large Rhododendrons and they have grown back more dense. Some cultivars tend to have a rangier appearance naturally that pruning, right after bloom, can help modify. Others may become sparse because the area around them has changed and no longer meets their needs as well, such as if trees have grown so large that there is too little light or if drainage patterns have changed.

If given more light they will have more growth and blooms. Can you prune some branches on nearby trees?

Large rhododendrons can be pruned as if they were trees rather than leggy shrubs that need to be 'revitalized'.

I have massive rhodies in my inherited garden that are 50 years old - planted when the house was built in the 70's. Most are in a lot of sun and are very dense, so Mike's suggestion to provide more light if necessary makes good sense. Native rhodies that are natural forest dwellers tend to be very tall and leggy just by virtue of their shadier location. But all will respond well to hard pruning to reduce height and make more compact/dense....at least initially. Light availability will continue to play a factor in growth habit.

Most species and cultivars will produce a large number of latent buds that are stimulated into growth by a hard pruning. But the plants needs to be healthy to begin with, it can take several seasons before it starts looking like a "normal" rhododendron again and resume flowering. But if still light challenged, new growth will continue to be sparse and leggy.

And as Mike noted, arborizing or removing lower branches to make the shrub into a more tree-like form can help to maximize the appearance even if in a very shady site.

Don't know if it's seen much where you are but I think you should check for downy mildew before cutting them back hard - many in my area have been made see-through by this. It has been my experience that when hard pruned significantly infested specimens do not recover as the mildew transfers to the comparatively small stump sprouts and may often kill them. Which finishes the already distressed plant off.

Downy mildew is indicated by yellowish patches on the surface of affected leaves, and on the under surface of the leaves it will appear as a whitish gray or even a bluish-gray mealy growth - symptoms similar to those of rust. The only true way to determine the source of the disease is with a magnifying glass or by consulting a pathologist.

https://www.rhododendron.org/v48n1p19.htm