There's actually a pragmatic reason to let the FARC become their own political wing.
Basically every single opposition political group for the last 50 years has been accused of being a political front for the FARC. This included the government sending hitmen against human rights activists, environmentalists, trade union leaders, teachers, the clergy, journalists. You name it, they shot or stabbed or poured acid on someone or blew them up and claimed they were FARC. In fact Colombia one lead the world in trade union leader murders. By "lead the world" I mean literally > 50% of entire worlds trade union murders were in Colombia up until a decade ago. They "solved" their labor disputes by mass murder of activists and claiming people who asked for pay raises or safe working conditions were FARC.
This is a reason they don't want FARC to come out of the shadows. With "ghost FARC" out there it's easy to label the entire left of the political spectrum as secretly FARC, people who want better working conditions or freedom of information? Those guys are FARC too! It's literally the ruling party's most powerful weapon against the opposition parties - we can accuse you of being dirty terrorist commies, then our death squads come for you, what you gonna do then? This effectively keeps Colombia as a One Party Terror State. Santos inherited the party and he wants to make his mark on history by changing that, but the Party Old Guard represented by Uribe doesn't want things to change.
If FARC become an actual political party, then saying "everyone is FARC" which is what they have been doing would be seen for how nonsensical it actually is, as it would be to claim the Tories in England are secretly working for "UKIP".