It's a fairly normal thing in plants. Even "evergreen" plants don't keep their leaves for ever, it's just that evergreen plants lose leaves steadily over the year rather than in one whoosh in autumn (except, as you've found, they may drop a lot if they're short of water, as a way of cutting down on water loss).
So when you have a plant whose stems get longer, the new leaves will be at the growing tip and the old leaves at the base, so it will lose leaves at the base, hence long bare stems with leaves at the tip.
Plants to a greater or lesser extent can put out new branches when pruned, or be propagated from "cuttings" - pieces cut off the end. It's 50 years since I've grown Philodendron, so I'm not an expert. But my logic would go: it's producing long stems without side branches, so it's wanting to reach new territory. Therefore it's probably got the capacity to root when it reaches somewhere suitable. Therefore what I would do is take one of the long stems and peg down the end into a pot of compost. Once it's developed good long roots, you can cut the stem and you have a new plant. The advantage over a cutting is that the developing plant still has the advantage of nutrients and water from the main plant. It's called "layering".
If there's nowhere you can put a pot, then you can put a handful of compost around the stem just below the leaves, and secure it in place by wrapping it in cling film - this is "air layering". You can encourage rooting by scraping the stem one side with your fingernail to produce a tiny area of damage. You can remove the new plant once the cling film is filled with roots.
Philodendron is recommended for places in the house with low light. It won't like direct sunshine, it's used to growing in the shade of other plants. And because it is used to growing among other plants, it hasn't had to acquire any defences against getting too dry. It may be happier somewhere else.