Is there any good reason not to let spells like Eldritch Blast or Ray of Frost target an object when other spells, like Fire Bolt, can?
Specifically with Eldritch Blast, you could use the special ability that pushes your target to move objects at a distance, and that could get very difficult to adjudicate when you're throwing out three blasts in a round. With that kind of
useful at-will magic, I'd be more prone to try and solve obstacles with an impromptu game of Portal than in the usual way, and that's not something that an inexperienced DM could deal with very easily.
For things like Ray of Frost, there's an issue where different objects may react in unusual ways. You probably can't harm a rock by freezing it, for example, but sufficient rules to explain that would be prohibitive in terms of complexity. Likewise for Chill Touch, however that one is supposed to work. To contrast with Fire Bolt, people have a pretty good understanding of how fire works, and there's plenty of reason why you might well
want to attack a rope or torch or something.
I don't know if you consider that a
good reason, but it's something. Ultimately, I would just recommend that the DM approach it on a case-by-case basis, and let those abilities
target objects but sometimes not have an appreciable
effect.