He is able to do this by having excess powers and impunity. You can't get MPs loyalty without state largesse to bribe or intimidate them. If he didn't control everything including cops, DPP, courts, NIS - he would be stopped from corrupting Raila, undermining DP, etc. State power corrupts daily governance.
Those motorcades and allowances he has given Raila are illegal. Deporting Miguna is illegal. Selective prosecution is illegal. Pampering of Gema farmers to win back support is illegal. It is the temptation to abuse power that is eliminated in parliamentary by separating GoK from sate.
Cops for example - he has compromised them by doctoring the law - NPS Act. He shouldn't assent laws.
DPP - if he didn't have appointing power for independent office there would not be selective prosecution. Kibicho might be locked up.
If we had parliamentary system - and Raila as opposition leader - enter into a path with Uhuru - Uhuru would still execute his reign of impunity - because Babu Owino will be his first defender. The problem is not the system of governance. It our unprincipled tumbo mbele politics. We are now one party system - Uhuru would get his way in parliament now. And in parliamentary system - nothing stops PM from corrupting the speaker and most of Mps to get away with everything - he can read a statement about xyz and that is it. How will you make him further accountable if he has the majority of Mps.