Salary vs. Hourly
It's not entirely true that no one in the fields you mentioned gets overtime pay. Generally a salaried (ie, "exempt" in the US) employee does not get overtime - the general expectation is that there's a certain number of core hours to be in attendance for, and a general minimum number of hours per day or week or month that the employee must meet. But there's also an assumption that some amount of overtime is part of the job, and not eligible for additional compensation. This generally falls into a loose assumption that engineers and other knowledge workers have enough autonomy and individual control over decisions that their jobs can't easily be gauged and budgeted to a perfect number of hours a week, and sometimes the employee picks up the slack with extra hours. In practice, this does get abused, and certain companies pick up a reputation for relying on those extra hours and way overbooking the workforce, making overtime a continuously mandatory thing.
Contractors, on the other hand, often do very similar work, but for an hourly rate. It cuts both ways - they may work fewer than the minimum in a week and get less pay. Or they may work more and the overtime can be more than just a flat rate, it's often time and a half. Different companies use contractors differently. A generally wise approach is not to put a contractor in a position where you can't live without them - if accountability in the long term is a must-have, hire a permanent employee. Contractors are usually considered "temporary" and in many US cases, they must be temporary or there can be legal ramifications to why they don't receive benefits.
For a company, the way the two types of employment are negotiated is part of the overall business plan. There's tons of nuances here and big companies generally have a defined strategy for these two roles. Smaller companies may be playing it more loosely.
There's cases, even, where employeed can negotiate an hourly setup for themselves, but this has it's drawbacks - generally hourly employees are not as highly ranked, and in a big company rank and salary vs. hourly are tightly connected. I've noticed questions here on TheWorkplace that suggests that there's no absolutes here - and that in some cases two employees with the same rank and job can have different pay setups, but I don't see that as the most normal case.
The easiest way to have this negotiation is before you sign on. Many companies will presume that when you signed on as salaried, you were OK with a certain amount of overtime. If the "certain amount" has gone beyond what's reasonable in your industry and location, then it may be another good time to restart the conversation.
The other trick is that most companies may not want to change a single employee's pay arrangement just to make one person happy. There can be a certain permissible unfairness in salary ranges, but when one employee gets overtime and the others don't this can cause a really big shift in how people behave. Realize that it's not just you - if they pay you more they may have to pay everyone more and that may be unsustainable. There can also be the issue of HR and how pay is organized from a paperwork perspective, but that's usually the lesser of the issues.
That said - another way to come at the negotiation is incentive pay. Many times, salaried employees are incentivized to get that extra overtime-requiring bit done. Incentive programs may be offered up front as part of a yearly program, or they may take the form of a post-success award. If you're new, it's worth asking around to see what is standard. If it's an after-effort award, it may not be so beneficial to go demanding money up front.
Incentives usually aren't connected to hours (ie, work 500 extra hours this year and get money...) - they are linked to performance and accomplishment of key milestones. It's worth it to ask about additional compensation for extra effort here... but try to put it in the most positive light - for example - "Wow, I see we have some big stuff coming up, and this is going to take a lot of extra work in terms of late nights and weekends. Can you explain what this success means for the company and the employees?" and if you're new "hey, I don't know how reward programs work around here? Can you explain>?" - to your boss or HR.
I've seen the case where in a really tight position for a given customer, a company can negotiate overtime for everyone. That happened a long while back on a high priority contract I was working. It was pretty easy to point to shifting requirements that led to tight schedules and overtime, and the contract was for a single customer. We were able to get a modification to the contract that let employees who'd worked 8 or more hours of overtime in a week get a bonus of a certain amount. This one is very much linked to the business - because there was a direct bill to the customer, the company managed to increase pay without taking a hit to profit.
I've also seen numerous cases of "work extra now, take time off later" policies. Most sane places will agree that you can push hard for a week or 3 but that people then need to catchup with family, health and home - so many companies may have comp time or extra vacation time in these cases.