To start with, a computer, especially back then, didn't necessary have a motherboard. Think S100 or Multibus. The SOL as motherboard based computer was an exception among S100 systems – and even it needed a separate memory board to be more than a limited terminal.
In fact, looking even a little bit further back, we'll see that not only motherboard based systems where virtually unknown, but memory was never confined to such. After all, how to put a core stack on there?
Single board computers where a development that only started in the mid 1970s when microprocessors became available. And even then, bus systems with CPU boards and separate memory boards where the norm.
Altair's S100, Intel's Multibus I, SWTPC's SS50, OSI's model 300/500 bus, Motorola's EXORciser, Digital Group's bus system and so on. Even first single board computers which could work as stand alone systems like SDK80 or MEK6800D1 did feature their company's system bus.
When independent and consumer focused machines became available the usually only had a minimal RAM capacity on board and featured either some expansion bus or a special memory connector for machine specific memory expansions.
The Apple II was eventually the first machine that left that path by offering full memory expansion on the mainboard. "Full" means here up to what the CPU could handle by default. At that time 16 Kibit RAMs where available and 48 KiB was just 24 chips (the rest was occupied by ROM and I/O anyway). Not much compared to what the 'remaining' components occupied. Like with the IBM PC, the decision for the RAM size possible was more driven by the assumed market, than any size limitations.
The size of RAM considered useful and/or needed depends a lot on the target market and application. A desktop machine didn't need much more than 64 KiB until the mid 1980s. A mini in contrast did – but then again, it wasn't motherboard based. Take this nice Olivetti M20 board of 1982. Clearly an upperclass (Z8000) workstation. The motherboard featured 'only' 128 KiB RAM (2 rows of 64 KIB). It wasn't really due to the board size (adding a few more rows of symmetric RAM would have been easy), but the assumption that 128 KiB is enough for most use cases. For expansion up to 512 KiB the memory bus could be used (black connectors).
But not every computer could do that [take memory on expansion cards].
The number of machines totally without a way for memory expansion was rather limited and usually within the extreme low cost segment – then again, even the ZX80 could do so. The only one that limited way beyond what's possible that comes to mind is the Atari ST series and the early Macs. Here again it was a use case decision – and a very specific market. No argument for a general case.
Bottom line: You haven't heard about, as it wasn't an issue – except in terms of money to pay for memory expansions.
On a side note, even before 1980 did mainframes feature backplane boards with sizes of 60 x 100 cm (make that 2x3 foot) – more than big enough for any mainboard an evil genius could have dreamed of in the late 70s. As a desktop computer its case may just have been less handy :))
So PCB technology wasn't stopping anyone from making huge mainboards filling 64 KiB with 2102-price in contrast would have (not to mention the needed PS :))