Why is that? Level3 is one of my upstreams and I've had two /23s from them for a few years. I'm pretty sure they still have some addresses available, although they might be harder to get nowadays.
Or, another option is that they're doing CGNAT on all of their client traffic.
Sorry, this is incorrect -- I'm a network engineer and I've been doing exactly this for years. I advertise "Level3 IPs" to other ISPs/peers, I advertise "non-Level3 IPs" to Level3, hell, I even "re-advertise" some of my customers' PI addresses (that they advertise to me) up to Level3.
Actually, I can advertise any prefixes I want via Level3 -- even yours (assuming you had your own).
n.b.: Now, some ISPs may require an LOA (or similar) before they'll accept certain prefixes from you but that's a (easily solved) procedural issue -- not a technical issue.