Many of them originally had a slate roof. You seldom needed vents with a slate roof because many a slate roof was not installed over an underlayment like a shingle roof. When you would go up into the attic of our old house, you could literally see daylight peeping through the slate layers, but it was watertight and we had no problems with leakage. (Heat leak, well that's another story). If any sheathing had been installed as an underlay, it could then be removed and was used just to strike a line. The art of laying a slate roof has been nearly lost and it's hard to get roofers who know the techniques and oftentimes they'll just slap down some sheathing and nail shingles to it, and skip the venting issue. We had to have venting added when we had our ancient slate roof removed, and our roofer, who was an experienced slate roofer reminded us of it. Some won't.

Our house originally had a wood shingle roof and the original skip sheathing is still visible in the attic. There are 1-2" spaces between the boards (laid horizontally over the rafters) which allowed the wood shingles to "breathe" in order to avoid rot, but it also allowed air to circulate in and out of the attic.

However, a few old houses (1900-ish) I've lived in had gable vents that were clearly original to the structure. So I guess there's no hard and fast rule.

Don't forget too some old houses had windows in the attic and those were often screened not glass to let air circulate.

And you know those systems worked just fine too! Before this house had a slate roof, it had a wood shingle one and we never had moisture problems. The problems arise when people decide to make old houses more modern and don't understand that those systems would no longer be adequate or functional if modified, or covered over, or closed. You just almost can't beat a roof installed the old way with a good quality slate. It'll be a good roof as long as you can keep people off it, and the nails don't rust for many, many decades, or sometimes generations.

Attics on houses built before the wide spread use of insulation did vent. The attic was not tight. Not only was the roofing (either slate or wood shingles) put on to allow air to move under and around the parts, but the eaves were not tight - you could see daylight through them.

Attics and framing need to vent because hot/warm air rises. In the spring, fall, and winter when that warm air hits the cold outside it condenses and makes water which will run down your rafters, drip on your insulation making it useless, and rot your frame. In the summer you have the opposite problem - the outside world may be hotter than your house, or you may wish to cool your house by expelling all the hot air out the highest point (your attic). Again condensation is the issue, along with keeping the hot air going up, out into the atmosphere.