If it is known that existing digital signature schemes like elliptic curve crypto or RSA are vulnerable to quantum computers, why haven't we already switched to Lamport signatures or something like that?
Quantum computers were not a realistic threat when these algorithms became standardized and widely deployed. As indicated in the comments by @Steffen Ullrich, hash-based signatures carry some disadvantages. The size of the public/private keys and the size of the signatures can be a non-negligible downside if quantum computers aren't relevant.
Additionally, some hash-based signature schemes are stateful. Stateless schemes carry a big advantage over stateful schemes in regards to implementation simplicity.
Is there something that prevents people from deploying this type of signature right now?
I understand that we still believe/know that nobody has a good working quantum computer yet but why not make the switch anyway?
For a cryptographic algorithm to be adopted for use in the real world, it needs to be standardized.
Hash-based signature have been submitted to NISTs call for post-quantum algorithms.. It is entirely possible that hash-based signatures will become standardized and recommended for use - assuming that they survive the analysis. The deadline for submissions was just closed (November 30, 2017) and it will probably be a few years before anything is standardized.