Well, for starters... dictionary hashing is too slow to be ideal. We don't swear off dictionaries, but we try to get our constant-time lookups without needing a hashing step when possible. The dictionaries can also lead to extra cache misses, since the memory often won't be compacted as you step through your data.

In most games this doesn't matter, but occasionally you have a frame time target and you need to take extra steps like that. Those extra steps can be hard work when they come up!