You might find the contractor has those conditions in their contract and they charge back to the company.
Some clarity here - it's not just the 'company' trying to stooge and save as much as they can, contractor labour is sometimes the easy option. I worked in mining for years during the 2000's heyday and at that stage the name of the game was getting as much coal out of the ground as you can. Company associates in the city would look at it like a paper game - where can we get trucks? How do we get hold of tyres? How can we organise drivers etc. A contractor like Emeco was in a great position because they had trucks ready to mobilise with drivers waiting. They could literally almost charge whatever they wanted. The unions on site hated this mentality and anyone who worked for them, but at the end of the day it was more profit which was better for the company. One scenario.
The other scenario is where a company decides to focus on their core business and outsource everything. A maintenance contractor would be better at undertaking maintenance than a manager who wants functioning plant. This happens a lot with cleaning companies. So send the feelers out, get some numbers back, and invariably argue why the more expensive company is a better choice (because at the end of the day the commercial department will pick the cheapest mob anyway). Contractor does it's thing. Then after the fixed term is up and the said contractor is comfortable charging heaps for all the overtime and training, the commercial department reviews the contract and goes "hey we could save a lot of money on this contract", and roundtable discussions result in a negotiation of rates more in line with what they think is fair. Hands clean, hundreds of thousands of dollars saved, and they go home thinking they've just justified their role. Where is the humanistic side in this justification? Because you can't put a number to it, it's left at the door.